When I don't feel inspired I tend to brainstorm by taking two stories and combining them. I once took a Bible story and rewrote it as a mafia tale. Combine The Godfather with Finding Nemo. Or what would happen if you combined Twilight with Saving Private Ryan?? Imagine a vampire braving a terrible war to find the human he loved. You could combine Dawn of the Dead with Bridget Jones Diary! haha It just gets you thinking outside the box and makes it seem like anything is possible again.
This is how we got a whole genre out of Pride and Prejudice and Zombies. On a less extreme scale it can be good for ideas, though... I'd usually say to be subtle about what some of the influences are, though. Like, you use very specific titles in your examples, but I'd keep away from crossing specific plots or you'll just be cashing in on that brief fad for Regency-style zombie killings, and it's just a fad, so getting lumped in with those sort of books will seriously kill any staying power a story could have. If it was done subtly and carefully, then it wouldn't look gimmicky, and could stand as a story all on its own. Eh, not sure how much you're talking about genuine fan fiction, versus just examples of stories from genres.
I'd combine, The Dark Tower series with, Back to the Future, Part 3 So we could go back to the end of Book four and have the last three books changed for the better!
Can I have three lol? I am currently working on a TV script that combines elements from Dr Who, Last of the Summer Wine and the Antiques Roadshow maybe I should throw Sky At Night and Top of the Pops in there whilst I am at it
Writing old stories in new settings is all we ever do as storytellers! I'd be careful about combining plots though. I recently got bogged down in a work because it was really two stories - so I split them up and they work so much better as individual books.
*Envisions the cast of Last of the Summer Wine, visiting the Antiques Roadshow, where the latest item being reviewed is a vintage police call box* Oh...the possibilities