Local multi-player and split-screen multi-player online as well.
The ability to play the online modes offline with bots (most Wii owners are not online and also good for noobs to get practice and build skill before getting endlessly slaughtered online) and bots in general if you're just online with a couple friends and don't want strangers but want more to shoot at.
No game destroying crashes in online
Better art direction
Less generic.
Use WM+ (chainsaw, some interesting alien weapons)
Make much better use of the ASE or leave it out. Nice concept, not so fun in practice
Some kind of filter technology for Wii Speak to filter out background noise
More fun - maybe more power-ups in online
Running... Everyone seems to walk. If you don't want everyone to just be running all the time, map running to 'chuk waggle. It'll be used sparingly or your arm will cramp. lol.