This first example might already be one that you've seen if you frequent r/custommagic, and may be the most widely known example on this list. Someone even went and wrote an article about it! Sword of Rage and Tranquility was printed in Blood Like Rivers, a set designed by ThatDamnPipsqueak. It entered the Revolution custom format on August 31st, 2022. The canon Sword of Forge and Frontier from Phyrexia: All Will Be One has a release date set for February 10th, 2023 (though was teased mid-January, which sparked the conversation). If there's one thing that both WotC and custom designers like, it's cycles, and the swords cycle is one of the most famous. Given red's big push for impulse effects in the past couple years, and impulse's natural synergy with playing additional lands (so you can avoid the feels bad of impulsing a land after your land drop), it's no big wonder that this coincidence occurred.

This next example isn't a 1-to-1 parallel, but there are enough connecting lines here to make one raise their eyebrows. Iroshi was printed in Viridian's Last Mission, a set designed by Chillbrain. It entered the Revolution custom format on August 31st, 2021. The Wandering Emperor came out in Kamigawa: Neon Dynasties, which hit the shelves on February 18th, 2022. I think the original design seed here was the concept of "flash planeswalker," and everything else fit into place after that. This explains the passive that lets you activate abilities at instant speed, and the fact that all of the abilities touch on tried and true flash concepts: protecting creatures, flashing in blockers, and removal. The colors may be different, but it certainly feels like they were both painted with the same brush!

Lands occupy a bit of a simpler design space; there just aren't as many novel effects that you can do if the card is not using non-mana abilities of some kind. I feel that this comparison is a perfect example. Wayfarer's Shrine was first made in Imperial Legacies, designed by Timespiraled. The card was a part of the Magic Set Editor Modern (MSEM) at its maiden voyage in June of 2017, and it's been a popular reprint in custom sets ever since (and still sees play today in MSEM, nearly six years later). Prismatic Vista was printed in Modern Horizons in June of 2019, almost exactly 2 years later. I think it's very fitting that both of these cards were featured heavily in modern-style formats!

Something else that happens quite frequently in the Custom Magic space, other than the functional designs being similar, are card names that end up being used by WotC later down the line. These stem from a variety of reasons; sometimes, a phrase is a somewhat popular idiom. Other times it is so simple that it was inevitable that it'd be used in canon some day. One very fun and unique example is Urza, Lord Protector from Kraken Cube, a project by Queen Emily which was finished in June of 2022, but had this particular design near the start of its development cycle in 2020. Brothers' War would later be released by WotC in November of 2022. While not a part of a larger format like the other examples here, it would be tough to pass this up, especially since they both also have a back face that share a name! The history behind this name can be found within the various novelizations of the MtG universe; Urza has had multiple titles throughout the years, including this one, making it less of a crazy coincidence and more of a fun prediction.

For a final bit of fun, I'd like to turn away from individual designs and towards other aspects of card design, in this case making Mechanics. It is not terribly uncommon for a mechanic used in the Custom Magic spaces to eventually find itself in the canon sphere of influence, with likelihood increasing the simpler the mechanic is. One good example is the Bleed mechanic from A Tourney at Whiterun, designed by HerziQuerzi. This was another set that was a part of MSEM's initial June 2017 release. Spectacle was first seen in the Ravnica Allegiance set, which came out January 2019. This definitely checks out as one of those "if it's simple it'll be used eventually" cases; alternate casting costs especially are a category in its own right that are very popular to explore in both canon and custom spaces (kicker, convoke, and prototype, anyone?).