
I have to say, I'm not a big... contributor, I guess you could say. I am a lurker. But I enjoy watching and following and lurking on quite a few communities.
The cap/ironman dreamwidth community is an amazing place and full of a lot of fun challenges for the fandom.
There is a wonderful prompt meme for drabbles and shorts, full of a lot of lesser known fandoms called fic promptly on dreamwidth that I wish I participated more in.
I found a really cool pillowfort community for marvel in general - a kinkmeme (so 18+ only, please).
I also follow a lot of fanartists and fanfic writers on patreon! There's Mauw, Ecchima, and lipeka, who are all amazing artists!
I'm sure there are others I'm forgetting, honestly. But tumblr, pillowfort, dreamwidth, and even discord (even though discord terrifies me) are great places to find communities.