The fandoms I signed up for are:
X-Men (First Class movie 'verse): I appear to be doing the time warp or something, as 4-5 years after this fandom peaked, I've fallen into it. My main pairing is Charles/Erik, though X-Men Apocalypse made me yearn for Charles/Erik/Raven. I specified First Class, not because it has to be based off that single movie, but because I don't actually enjoy rivals pairings normally, so the early 2000s Charles/Erik is less interesting to me, than the closer relationship we've seen in First Class/Days of Future Past/Apocalypse. I'm not opposed to stories that are set in the earlier (later, in canon) movies, just it's a bit more hit or miss how their relationship is portrayed. My favourite tag in the fandom is Alternate Universe - Still Have Powers because I love stories that explore how the world would react to mutants and the powers that they have but the actual canon is so intense, goes to such an extreme. These AUs generally have more of a middle ground.
Daredevil (TV): Ugh, my stupid avocados. I've got some of my faves bookmarked on AO3 if you'd like some guidance. My favourite trope in this fandom is Matt and/or Foggy desperately in love with the other and not realizing that their feelings are returned. Matt/Foggy is my primary ship, though after seeing S2, I also believe Matt and Karen deserve each other. Claire is amazing.
Marvel comics 616: I'm mostly interested in Steve/Tony centric stories from comics. Though Jan is my fav Avenger, so anything with her in it is amazing! I also have some recs on AO3.
MCU: My main pairings from MCU are Steve/Tony, Clint/Coulson, Clint/Coulson/Natasha.
For Steve/Tony, I prefer the stories that build off the comics canon for their relationship (because I admit they are not the strongest pairing within the canon). I have spent most of the year in denial about Civil War (some of the actions in that movie feel like a violation of the characters), so you're probably better off avoiding stories related to that movie. I also hate fanon!Bucky, so you're better steering clear of stories where he has a bigger part. My Steve/Tony MCU recs (these are pretty sparse)
For Clint/Coulson and Clint/Coulson/Natasha it's all about the competency porn for me. I love team Delta, I love them watching out for each other, I love them supporting each other, and I love them kicking ass together. My Clint/Coulson recs
Agents of S.H.I.E.L.D. I actually find Coulson kind of boring in AoS, and watch it mostly for Daisy (Skye) and Fitz these days. Anything about Daisy growing into the person she is today, femslash with Daisy/Anyone, are amazing. The only pairing I refuse to touch is Skye/Ward. And pretty much anything with an apologetic Ward is a no-go for me.
For Fitz stories, I love his friendship with Jemma. I prefer them as friends, but his desperation to save Jemma last season did a pretty good job of getting me on board the Fitz/Simmons train. I also like Fitz and Mack (and Fitz/Mack).
And if you wanted to combine my favourites, stories about Daisy and Fitz would be amazing. I especially would love a follow up story to their interaction in 402 where they spew feelings at each other.
Luke Cage: It's probably too soon for there to be much Luke Cage fic, but I would adore stories about Mariah. She was the character that stood out to me most. How she grew up to this life of crime, but what she really wants is to make Harlem a better place, but she keeps falling into the criminal underground. I love how Shades is so into her. My dream fic from this fandom would be a Mariah-centric story where she takes over Harlem and has femdom sex with Shades on the side. :P
Since I'm listing older fandoms, I always want more Terminator stories. I fell in love with the franchise through Terminator: The Sarah Connor Chronicles and I adore Cameron. I also have a thing for people growing into heroes so love John Connor's characterization from SCC and T3. My ships are John/Cameron and John/Kate, though this fandom is the one I read the most gen in. I also adore crossovers with Terminator. If you can find good crossovers with this and any of the other fandoms I've listed, that would totally make my day. My Terminator recs are on my pinboard.
Things that I love (an incomplete list): Rule 63, identity porn, trans characters, world exploration, found family, pretend dating (really, most harlequin-type storylines), angst, AUs, origin stories, power dynamics (including D/s and BDSM), emotional punches in the gut, and way more than I can think of at the moment.
I don't have many no go zones but cannibalism and killing people in a fetishized way are really hard stops for me. Related to the previous point, I mostly can't deal with zombies and am finding vampires harder to deal with too. Detailed gore on a large scale also makes me queasy and I'm not a fan of horror.
I don't enjoy stories whose plots are driven by homophobia, and I've been finding talk of conversion programs kind of triggery lately.
I'm also kind of hesitant about stores with cheating? If the cheating is actually addressed in the story, I'm generally ok with it, but if it's handwaved or something, that tends to make me sad.
Character-wise, I have no interest in Ward-apologists and find that even stories that feature Ward very much, that are set before the reveal, don't do it for me. I'm also really wary of Loki-apologists and, as mentioned above, don't enjoy fanon!Bucky very much.
Note: While I wouldn't necessarily list these as things I love, but since I see a lot of other people listing these as things to avoid, I will say, I don't mind unhappy endings and/or death fic. Related to my like of emotional punches in a gut, I like when a story can make me cry. I don't mind sad events or endings so long as the emotions ring true. So if you have a story that you think would work that has an unhappy ending or character death, go for it! I am well known as an angst muffin after all.