An intro for a comedy fanfiction should be comedic as well. It should summarize a crazy event, or whatever you're writing about in a way that makes the readers want to know how the character got into that crazy situation or event. There are a lot of ways to take a comedy (or crack if you must) in the first few sentences. They tend to work better on me when they open with hard hitters though.
I don't like to be too descriptive, you need to leave room for imagination so the reader absolutely has to keep reading to find out, but it should also tell enough to draw the reader in so they can connect with something instead of just reading a random blurb that doesn't really make any sense.
A few starters for a comedy intro would be -
This is not how it was supposed to happen.
You ever have that moment when you realize your friends are absolute children and make your life harder than it needs to be?
Sometimes {insert character name] makes [insert pronoun] life harder than it needs to be.
It was not [insert character name] fault
A flash-forward scene without context before going backwards. True Crime style.
The intro actually doesn't matter as much as the rest of the fiction surprisingly. It should introduce the reason someone needs to be cheered up, or why what's happening is happening and who's involved. This can be utterly unassuming because to me the most important part of a fluff piece is the body paragraphs.
The intro should set up the background of the event that is so fluffy, or the event that caused the need to be comforted.
A few starter sentences for a fluffy intro would be:
At the end of the day all [insert character name] wants is to cuddle with [insert character name]
[insert character name] cuddles were the best, and you won't convince [insert character name] otherwise
[insert character name] was looking forward to [insert date/time/event] since they were going to get to spend some time with [insert character name]
Just start at the beginning, sometimes that's the best kind of intro for fluff pieces
The introduction is the most important part of this, it brings the reader up to speed on how the character feels at the moment (which evidently isn't that great if it's in the angst department) it describes how the character got there, or the moments leading up to this particular sadness which may or may not be the originating event. A character doesn't necessarily have to already be in their sad girl era at the beginning so you can make it happy, but personally, I would recommend alluding to the incoming hurt.
The intro should be emotional and raw, including hate, guilt, and sadness, directed at others, or even at the character self (if angst hours have already begun) otherwise I would recommend a benign intro that leaves the readers completely unaware of the punch in the face they're about to get.
A few starter sentences for an angsty intro would be:
[insert character name] wasn't feeling too great, but what a surprise, they never were.
Depressions a bitch, and [insert character name] would know that first hand.
The urge to cut didn't just appear, it came slowly, like a fog falling over you, blinding you.
If it's a fic that transitions into angst (hard hitters) then I would say to make it extremely happy to really dig into the readers heart
Or happy, but notably not as happy as they could be
A romantic intro brings up the feelings of the two or more characters (depending on what you're writing) and how they're directed at the other. It shines a light on the behavioral changes between the characters, and when they realize their affection for each other, and then (usually) brings up their immediate removal and pining.
Now you can write three different kinds of romance, and I'll highlight them and their specific introductions.
PINING
This intro is the beginning of the relationship piece, or how the relationship came to be filled with endless pining and wishing the other returned the notion. This intro should be yearning, yet also self-deprecating due to the character's belief that the other could never feel the same. It should also highlight how they consider the other adorable or attractive.
ESTABLISHED
This intro is about a relationship that's already sailed. It can be about a date, an anniversary, or something completely random that involves the couple. The intro should highlight how happy the characters are together, a soft moment together, or perhaps the soulmate vibes they give. If it's an anniversary bring up a gift or how they got the idea of the gift or date, and even all three. Established opens up the possibility of writing without struggling with the pre-relationship difficulties while also allowing the writer to make Hurt/Comfort and figure out the new dynamic between two characters. Writing an Established couple means you can skip the pining or the relationship dynamic beforehand and get straight to the romantic/queerplatonic relationship.
PROBLEMATIC
This intro can be about any kind of relationship at all, but the point is that they're having an extremely serious issue. Toxic behaviors, impending breakups, moral disagreement, emotional damage, you name it. This intro can highlight how unhappy and fed up the characters are, show a refusal to admit the terrible position a character is in, or the beginning of said problematic relationship. It doesn't exactly have to be clear about whether or not the relationship is less than healthy at first, but if it does I would recommend highlighting the bad behaviors of whichever (or both) character[s] is the "problem" in the scenario you're looking to write.