Street Sharks.

I think samurai champloo would be amazing.

Honestly, I think most series are better off as a TV anime, unless the source material is that short.

One that stands out is Umineko, one movie per episode would work very well.

The next three Kaiji arcs after Season 2 could be done in a movie each also (Mahjong Minefield, Test of Friendship and One Poker)

Das Finale Parts 5 and 6. Part 6 has the potential to eclipse the K-On movie.

This is kind of an odd question. Everyone will be biased to what they like themselves, but if I have learned anything lately its that the good stuff you think would make tons of money gets locked away forever, and the same mainstream normie stuff is what gets pushed hard and the normies keep buying it up.

No Appleseed, no Bubblegum Crisis, no Perfect Blue, no Silent Mobius, no Record of Lodoss War, none of the good stuff like those.

At this point, those IPs have next to zero brand recognition, and the companies that own them see them as too high risk to be worth doing anything new with them other than whatever is legally minimally required to keep the IP and trademarks from becoming public domain. Either that, or they make a live action remake that turns out to be worse than ExARM's animation quality.