Vehicle physics that don't suck, as well as an improved handling model. Something similar to Carmageddon 2 would be awesome.
Also, dynamic per-pixel lighting and stencil shadows please.
EDIT: Something I've really wanted for a while is the ability to use tracker files for music, prefferably with support for the .mod,.s3m and .it formats. 16-bit quality upsampling (like ModPlug Player) for music modules using low-quality samples would be really nice.