
Also there could be more bridges (where you can't only send units on it but under it) and perhaps new factories. (And, of course, like you said, the old levels and videos). The main menu and the ingame-menu could also be improved (and get much smaller so that you can see more of the game screen).
Also it would be absolutely neccessary that the game supports multiplayer (with cpu and real players) on internet or LAN. It's already great fun to play together over the good old serial DirectLink Cable, but it would be much greater to game with more than one enemy and to have allies or sth.
And it would be amazing if the game music wasthe same as the old Z music, played as MP3 and having the sound of the really best MIDI synthesizer i ever heard of: Creative AWE64 Gold (although it has a little hissing and bad sound quality i LOVE it!!) It wouldn't be Z anymore without that sound.