And here is another storyline I won't write, because I have already two of this kind in my mind that I absolutely want to write, and if I tried this one too they might end up being too similar. And yeah, I like to use things that have been done many times, but at least, I do it with characters almost no one thought about.
Of Lost Wizards and Strange Travels
Fandom: crossover Harry Potter x Lord of the Rings
Timeline: post-Hogwarts, starts while Frodo, Sam, Merry, and Pippin are travelling accross the Old Forest. You can do a mix of the films and the movies, as you please.
Main characters: Fleur and Bill Weasley, Sirius Black
As the war is finally over, Fleur and Bill can finally leave on their honeymoon. While packing up, Bill inadvertently takes Harry's two-way mirror ( How it came to be here, you choose ). The journey goes well, they go to see the Delacours before leaving for a world tour. But while in France, something goes awry ( then gain, you imagine what ) and next thing they know, the two newlyweds look at a great and old forest, where two beings that looks a bit like children are battling against an old willow.
Oh, and that's strange, but Sirius blinks back from his peaceful Death, like, literally blinks and then is back and ends up as a ghost haunting is mirror. Why? Because Sirius is awesome, and I like him ruthless, yet a powerful genius in all the magic Arts, Dark Arts included. He doesn't particularly like causing pain, but it doesn't rebuke him, and sometimes anger can lead him to pass the limits of the Dark Arts ( never the Unforgivables, though ).
Somehow, the witch and the wizards ( not that Sirius has nuch of a choice, being stuck in a mirror and all ) joins the Fellowship.
Their magic has to adapt itself to Middle-Earth and the local magic, like sme kind of hybrid magic, maybe, for it's too easy to play the card of the supra-powerful wizards from Earth who can do things completely incredible even to Gandalf. After all, HP has a lot of spells and curses and all but it's most of the time to use on one, maybe two opponents at a time, not an army. Balance, I beg of you.
Oh, and Sirius can travel from his mirror to another one, as the portraits can, if they are in the same room. And I don't mind if you get him out of the mirror at some point, like in a tight spot when everyone is fighting for their lives and the frustated ghost wants to help so much he somehow break the mirror. I told you Sirius is powerful and awesome. Just don't free him too soon, because we need angst. It's angsty to be stuck in a mirror when your friends are risking their lives.
You can have Sirius be the genius of the group, able to explain to his younger friends most of the differences between magic on Earth and on Middle-Eart, and to teach them others ways to use magic in battle... I'd be most grateful if you did.
Use the surprise from the quarter-Veela-thing, Sirius being freakingly handsome and Bill being quite handsome too even if Greyback did quite a number on him. The elves will be surprised.
The Hobbits aren't too suspicious, Sirius eventually bounds with Boromir ( you see if you wat to save him or not ) and Aragorn, Bill with Gandalf and Merry and Pippin, and Fleur with Legolas and Gimli.
Whether you want to assume that Westron is English or not is up to you. Anyway, Fleur will have some difficulties communicating at first.
As I said Sirius is clever, strong, devilishly handsome ( or should I say of a suspiciously elven handsomeness? ) and purely awesome.
Bill is a nature enthusiast and appaled to hear that dragons are evil around here. Still, you can't get him to find a baby dragon and adopt him. You can't. He's good at tracking and dealing with traps ( Curse breaker in another life, man ).
Fleur is clever and creative with her magic in battle as in normal life ( normal for the Fellowship, that is ). She's stubborn about fighting alongside her husband, and pretty scary when angry ( a quarter Veela, remember? )