I'd love to have 4-player split-screen, to me local multiplayer party is way more fun than playing online.
However I think that's unlikely to happen in GT because of hardware limitations.
I wouldn't mind GT running on 720p & 30fps on split-screen and running 1080P & 60fps on single player, that's better than not having split-screen, but I really doubt any game developer would consider doing something like this.
Also, it would be awesome if split-screen races had AI opponents and all the options available (car setup, damage, tire wear, etc). Only 2 players with no options is really boring.
PD should also include lan multiplayer.