Humans. Humans are much like they are in every other campaign setting. Baker's text talks about a (relatively) recent expansion of humans over the continent of Khorvaire from their original homeland in Sarlona from beachheads in the east, what is now the Lhazaar Principalities. This means that the ethnic diversity among humans is way overstated to the point of absurdity. Although the arrival of humans from Sarlona is all well and good, it really doesn't matter too much except as used to explain broad patterns. If in the Remixed version of the setting we had Sarlonan humans expand westward from beachheads on the eastern edge of the continent, gradually overwhelming culturally and linguistically the humans that were already here (because in my heightened humano-centric assumption, there have to already be humans here) then what you'd expect to see is that a relatively common phenotype is especially common further east where they spread from, and further north where they would have less admixture from existing populations due to the less productive climate. The further west and south you go, the more you'd come across a phenotype that was already here before the Sarlonans arrive. In very broad terms, I'd say that the Sarlonans were Nordic in their phenotype; taller, blonder, paler of skin and eye, and the ones already here were more Mediterranean in phenotype; shorter, more gracile, dark hair, brown eyes. Much like the spread of the Indo-Europeans across Europe in the Chalcolithic and Early Bronze age via the Corded Ware archaeological culture and its derivatives over a population that was heavily weighted towards Mediterranean Neolithic farmers.
This is kind of a moot point now; it's hardly like the Indo-Europeans recognize in themselves cultural brotherhood or anything and the same is true for Khorvaire, but as the Sarlonans dominated, subverted the culture and language, and established themselves as a superstrate over the original humans, you can see the result of that in their genetics and phenotypes. In some cases, the original Sarlonan phenotype has largely disappeared, though—as it has in Rome, where most of the Emperors were described as having blue eyes and blond or red hair, but everyone now looks like a stereotypical darker, Mediterranean Italian. However, in a vague sense, there is a recognition that there are two basic, broad types of humans on Khorvaire; the so-called northerners who's ancestry is more recent in Sarlona, and the Old Folk who were here already when they arrived.
According to the rulebook, humans dominate several of the Dragonmarked Houses, including Cannith (fantastic manufactured items), Orien (transportation and shipping), Deneith (bodyguards and mercenaries), and Vadalis (livestock.) In addition, I'm making Kundarak a human House, since there are no dwarves (banking), and Medani (a kind of private eye and international federal marshals all rolled up into one.)
Changelings. There's actually little reason to change much about changelings from as presented in the book, other than to assume that there are fewer of them. Although one never really knows exactly how many that there are, because given their abilities, they can easily foil any efforts to keep tabs on them anyway.
Dwarves. There are no dwarves in Eberron Remixed, and the roles that dwarves would have played are taken, instead, by humans. The Mror Holds becomes a Swiss-like human nation, therefore, and any other dwarven NPCs are simply human of whatever cultural ethnicity you need them to be from.
Elves. There are no elves in Eberron Remixed. The role of many elves will be taken by the seraphim, but the reality is that the seraphim are an over-caste ruling over a largely human base populace, even in nations like Aerenal and Valenar, and there are considerably fewer of them than there were elves in Eberron As Printed. There are, broadly speaking, three cultural groups of seraphs, broadly corresponding to Aerenal, Valenar, and the Khorvaire seraphs who are culturally pretty assimilated to whatever human nation that they live in. And conversely, there are significant numbers of humans who are culturally Aerenal and Valenar based, even though they aren't the racial caste that has most of the power. Valenar in particular is a somewhat meritocratic place, and humans of Valenar culture who are capable warriors can rise to social prominence equal to that of any seraph. Three Dragonmarked Houses are known among the seraphim, Phiarlan and Thuranni bear the Mark of Shadow. Both specialize in entertainment, arts, and other such pursuits, although the true bread and butter of both is espionage, assassination and other, darker pursuits. Although the Dragonmarks themselves only manifest on seraphim, both houses maintain a large population of humans as trained specialists as well. House Lyrandar is one more dedicated to shipping and transport, and is famous in particular for its airships.
While not native to Khorvaire, the drow of Xen'drik are also replaced by populations of kemlings.
Gnomes. There are no gnomes in Eberron Remixed, and their place is mostly taken by the Hyperboreans. The Hyperboreans were originally from far to the north, in the lands of Zobna and later Lomar, which are now completely lost and presumably frozen over in the Frostfell. Their first beachhead on Khorvaire was on the northern coast, and the city of Inganok on the island of Farlnen, far to the north and east, where they mine onyx and basalt and have a decently established trade network with the Mror Holds and Karrnath in particular. The majority of the Hyperboreans moved further south, however, in ancient days, and established the nation of Zilargo. Hyperboreans manifest a unique Dragonmark too, and House Sivis is the one that they control. It serves as communication and documentation; the "dragonmarked telegraph" system (which I need to either find the name for or come up with one of my own) as well as serving as a kind of neutral "notary" for treaties between nations and other important documents is their stock in trade, although by nature, the Hyperboreans are often involved in mafia-esque concerns, and often serve as rivals of sorts to House Phiarlan and Thuranni in some of their areas of mutual interest.
Half-elves. There are no half-elves in Eberron Remixed. Their place is taken by just humans or seraphim, and the two houses are already discussed above in the Human section. House Lyrandar is the exception; being a half-elven House in the book, but a seraph dominated house in the Remixed version of the setting.
Half-orcs. There are no half-orcs in Eberron Remixed, although Orc is itself a playable race (as is Goblin.) That said, Half-orcs are not, curiously, replaced by orcs; my orc race rather takes the place of hobgoblins in Eberron material, most of the time. The orcs, especially of the far eastern lands like the Shadow Marches, the Eldeen Reaches, etc. are instead replaced by Atlanteans, descendents of Atlantis who were caught off of that doomed continent when it sank and found themselves stuck on Khorvaire in ancient times. While they seem like primitive and especially xenophobic humans with strikingly unusual physical features, in reality the Atlanteans have a number of unique traits to them; a heritage brought on by the curse that doomed Atlantis, even though their immediate ancestors survived the fall of that land. While most Atlanteans do not mingle with the other races on Khorvaire, those who belong to House Tharashk do, to some extent. It is one of the only dragonmarks to manifest on more than one race; it can be found on Atlanteans and local humans of that same area both. While the kumbaya altogether type personalities see that as evidence that Atlanteans are really just misunderstood brothers after all, for the most part the Atlanteans would torture, kill and eat anyone fool enough to go visit them preaching that message of unity. That said, House Tharashk, who's dragonmark gives them supernatural insight into investigation and finding, has managed to make a fair bit of money because they seem remarkably capable of finding treasure lost in past empires, dragonshards, and other items of exceptional value. That said, their involvement in the politics between the Houses is relatively minimal.
Halflings. This is the perfect example of where Eberron tried so hard to make halflings different and interesting to the point where it just begged the question of why even bother making them halflings at all anymore then? In any case the Remixed version of the setting doesn't have any halflings at all, and the nomadic, plains-dwelling dinosaur-domesticating people are Jann, not halflings. Otherwise, they are considerably more rare across the rest of Khorvaire than halflings are, nor are they as involved in crime as a rule. There are two Dragonmarks that manifest on jann, however, so House Ghallanda (hospitality) and Jorasco (healing and medicine) ensure that at least a few jann representatives are present in any settlement of decent size. Of all of the Dragonmarked houses, these are the two that are most welcomed with the least suspicion by the non-Dragonmarked, in spite of the obviously foreign appearance of the jann themselves, because it is generally seen by most that the service they provide is of benefit to the entire community. The jann of these two Houses have fostered this as a public relations coup almost over the other Houses; while they may not be the most wealthy, or rub shoulders with the most powerful, they are the most entrenched with normal people and have their support to the extent that few even of the most cavalier of the kings of Khorvaire would want to be seen publicly crossing them.
Kalashtar. The kalashtar are pretty much exactly as described in the book. Although their origin is on another continent, I have no real interest in pursuing cross-continental adventuring, to be honest with you. Many kalashtar who are here are either refugees or the descendants of refugees, fleeing the oppression of the Inspired of Riedra, who also maintain diplomatic relationships with many kingdoms in Khorvaire. Few have any desire to return to their embattled homeland of Adar, however, and most seek to make a new life for themselves and possibly even their communities in Khorvaire.
Shifters. Shifters are also pretty much exactly as described in the book. There's no reason to fix what ain't broke! This is one of the few races that I'd actually maybe see increasing slightly the percentages especially in the Eldeen Reaches where they should have a strong plurality; I tend to really like them and would like to see a few more of them. While the book says that they don't have any lands of their own, I see them as having autonomous (at least de facto, even if its not recognized by any other power) tribes in many of the larger forests and wilderness areas and the northern and western portion of the Eldeen Reaches essentially is the shifter homeland and greatest concentration of them even now. It should be noted that while Dark Fantasy X has shifters (basically) it uses another name for them; woodwoses. For Eberron Remixed, I think it makes sense to revert to the Eberron name.
Warforged. Warforged are also pretty much exactly as described in the book. Another ain't broke; don't fix situation.
Other races. Eberron always did have a tendency to take a handful of "monstrous humanoid" races of more or less PC level potency and give them a more PC-like approach. The Dark Fantasy X ruleset that I'm using does feature a few "monstrous humanoid" races, at least as they're considered in D&D, as PC races too, but others are still just monstrous humanoids and shouldn't be encouraged for play. If these monstrous humanoid races weren't exactly encouraged in Eberron, they were at least specifically allowed, kinda. Orcs and goblins are particularly common in their newly forged nation of Darguun, and their legacy of the ancient empire of Dhakaan before the arrival of humans of any type means that they can be found in relatively small numbers in many places across the continent. The Daemon Wastes (yes, I have updated the spelling to match my monster list) is another homeland for kemlings, who are spread much more across Khorvaire in slight numbers compared to any of the populations that they're replacing. The Daemon Wastes history is, in fact, considerably changed. Although I don't want to get too far ahead of myself, I'm going to suggest that Ohr Kaluun, the tiefling homeland near Sarlona was matched by its twin kingdom of Bael Turath, and the wreckage of that kingdom is what the Daemon Wastes is.