There is little on the gaming front that WinXP does that Win2k doesn't. Come to think of it, there's nothing that XP does that 2k doesn't as far as Windows games are concerned. I mean, the same version of DirectX is available for both, and that's what matters. So, support Win2k/XP/whatever, which ties nicely into the Xbox if you want to head that way (as (IIRC) the Xbox is a stripped-down Win2k kernel). Also, just because you can use all sorts of fancy effects doesn't mean you have to. There's no reason to alienate those of us with older graphics cards/systems either (i.e. GeForce 4) - look at what Uplink acheived with simple 2D graphics.

Anyway, enough of the rant about system specs.

I'd keep the side-scrolling type of gameplay. There's enough FPS games around as it is, and to create a FPS Jazz would detract from the current style of things. By all means make it 3D, but let's not make it just another Doom game.

On the multiplayer side, the current game types are pretty good, but it'd be nice to have some sort of scritping system to create extra ones. Then again, the lack of a scripting system hasn't stopped people coming up with some strange levels by (ab)using warps...

With the rest, I can't really think of anything that hasn't already been said.