I talked about Fan Trailers last year. Mash Up Trailers are something different, basically the Audio of a movie Trailer put over different visuals, usually from an Anime giving it a Pseudo Fan Dub quality.
I first discovered them years ago via some Mash Ups of The Dark Knight to Code Geass. Then last year I found that Dual of The Fates Utena AMV, which was done to the Music Video giving it a trailer like quality.
The only one not to Animation I've really enjoyed is the Pretty Little Liars Suicide Squad one done by one of the ProsWatchPLLToo people.
Then I started seeking them out more regularly and found quite a few. I watched some of Code Geass to The Dark Knight Rises, some Death Note to the Dark Knight films. Some Code Geass to the Star Wars Prequels ones, as well as Evangelion to Star Wars. And Some Mean Girls ones, mainly one to Lucky Star. They are included on my Anime Playlist.
So I want to mention some Mash Up trailer ideas I'd like to see that haven't been done yet. And that I'm not talented enough to do myself.
Smile Pretty Cure/Glitter Force to The Dark Knight. With Joker as The Joker, that's obvious, that's where this idea starts. Reika as Batman, since she's the one with a one on one confrontation with Joker. Pop as Alfred, and Nao as Rachel Dawes since she's who Reika is usually shipped with, (according to TVTRopes anyway).
Returning to the subject of Code Geass and Star Wars, I'd like to see that Padme focused Phantom Menace trailer that has already been done to Code Geass once, done to Akito of the Exiled, with Leila as Padme. And I'd love to see some Gundam/Star Wars mash ups, if they exist already I'm having trouble finding them.
Related to that would be going Gundam to Star Wars, for me the most meaningful would be 00.
And another Anime that could be fun to see done to Mean Girls would be Yuru Yuri.
And as I said before I'd like to see some using Noir and Madlax.
Update: I’d really like to see Yuki Yuna is a Hero done to some Star Wars trailers. Those scenes of Karin practicing on the beast with her wooden swords particularly feel made for John Williams music to me.