Then I suppose we're left with Story DLC (or any other kind of paid DLC). This would depend on when they release, though.
On the topic of Steamworks, I'd like partially-full integration, myself. If that makes sense.
Stuff like cloud storage does not seem to be necessary for GTA Online (since player character info is stored on Rockstar's servers), but as far as I know, story mode does not have cloud saves. So having Steam cloud storage of SP saves would be great.
The achievements as well, obviously.
The big thing I want is trading cards. V is a really good game for those. Just imagine a big ass V badge on your profile with a fitting background. Come on. That'd be good.
Everything else I don't really care about. I sure don't want VAC infesting Online. Heck no. Keep that thing out of games without dedicated servers, please. It works best in games that do have them.