There were plenty of new features in Vice City. Not enough to call it a sequel (which is why they didn't, duh), but plenty new additions over GTA3. I'm very sure that there will be plenty of new additions to GTA4.
And even if there aren't, I'm still going to buy it.
As for online, OK. Those features I could maybe see. By some people want a MMO game, where everyone just runs around killing one another. And that's just gay. But playing with one or two buddies for some sort of online mission stuff wouldn't be half bad.
But I still don't see the big deal over online play.
Back to the original question. Perhaps some destroyable/interactive objects? Like, maybe there could be a giant statue, and you can knock it over with a carefully placed rocket. Then you can climb up it to get to some hidden package or something.
Another thing I'd like, is the ability to disguise yourself from gangs. If I'm in a cop car or something, I really don't think gangs should be able to recognize me. That way, if you missed something in an area where you're "marked," you can go back with a sense of safety.
I'd also like the planes and boats to handle better. A lot better. Moreso the boats than the planes.
And of course, more interior areas.