Don't forget: the primary function of fandom and fanfic is to have fun. If making OCs and mixing fandoms is fun for you, then no reason not to make OCs or mix fandoms. (I posted an x-over in October that was a mashup of two fandoms that didn't make sense together and it worked better than I expected.) The people that are interested in them will find them. Do what makes you happy. You don't have to write for anyone but yourself.
Is it people commenting on your work or just a trend you see happening in discussions of your favorite series?
A trend in discussions and something that's happened in the comments of friends' work. There's been a lot of harassment and bullying from people who like the main ship and they're starting to get louder again so I'm hesitant to even touch on certain characters or topics or opinions just cuz I want them to ignore me. But...like...I'm also really sick of pretending I'm cool with their enforcing certain dynamics and things as "correct" when...there's no such thing in fandom. It's supposed to be a place we can all enjoy, not a hierarchy of "I'm correct because I belong to the majority opinion and all of you are stupid and wrong for believing otherwise". I've done spite writing in the past and it worked really well for getting those negative feelings out, but...like...it also kinda made that fandom less enjoyable because I just equated it with having to fight instead of enjoying things.
I think it's better if your friends confront them on their own than you stepping in. Its their story so they have the right to police it.
No? Why would you want to do that? Is the fic orphaned? Did the author state they were allowing others to finish the story?
Yup, I have. It's someone I have a good rapport with though, and they insisted upon it; I'd not have done it otherwise. I've also given readers of one of my own incomplete stories carte blanche to do the same (though I don't think they ever took me up on it).
I did for a fic that got deleted when its hosting site abruptly shut down; the author had given me a copy of the fic before then and never resurfaced after the site went down. Didn't post it, though, cuz I didn't know if I was allowed to and it didn't feel right to post it without permission. If it's been a long time since the last chapter was posted, you could always send the author a private message saying you enjoyed it and are looking forward to future updates; if they say the fic's abandoned, you could ask for permission to finish it. Some people are cool with that. Some people also have permission statements on their profile saying what people are and aren't allowed to do with their writing (for example, I don't mind if someone wants to do fan art or inspired by or pod fic of my fic, but I would like people not to write prequels/sequels/spin offs of what I write), so that's worth checking as well. Some authors will also tell you what their plans for a story they abandoned were if you ask, so that's also worth a try. ^^;