Hey!
First off thanks for reading.
My suggestion is to allow switching between Vertical and Horizontal split-screens. This is to make life easier for people running on multiple monitors (generally setup side-by-side) to utilise a full monitor to each player.
Also having better support for multiple monitor setups again so that each player can have a full screen to use.
Granted this is probably a niche sector of your player base and perhaps not worth the time to implement but it's a feature I think would be great.
Many Thanks,
Ewok