* Way more actual crime-based side activities. Drug dealing, weapons smuggling, robberies beyond just convenience stores and armored trucks, vehicle thefts, basically pretty much everything that was already available in GTA IV. Trevor has several, but they're way too few, and Michael and especially Franklin need to be able to do some, too.
* Open up those interiors, and bring back the restaurants dammit!
* Give us more strip clubs and bars. It's so weird to me that LS only has one strip club you can go in.
* The property management. This needs major help. One, let us go inside the buildings we buy. Two, let us do the missions at our own pace rather than introducing this game's version of "HEY NIKO IT'S ROMAN LET'S SHOOT SOME POOL" that's so much worse, because you can't even put your phone on sleep mode to avoid them, at least when I tried to (it's like they decided to include a feature that's exactly as annoying as everybody makes the friends system in IV out to be!). And three, fix the whole income system so that our investments feel worth it.
* More safe houses. I know M, T and F each have their own homes, and I actually like that a lot, but come on, let us buy other houses that we may prefer over their default homes for one reason or another, or just to have places to crash all over the map. And on Trevor's side, let us keep the beach condo after "Hang Ten". I really liked having a place for him to crash on the beach, and didn't like the Vanilla Unicorn half as much. And how twisted would it be for him to keep that place for his own after, uh, evicting the previous residents?
* Let us get any hair style at any barber shop. I think Trevor looks best with the Big Mustache and like to get it for him ASAP, and it's annoying to have to drive all the way to LS just to get it. I can understand clothing shops selling some clothes and not selling others, but hair styles are hair styles. I hated this in San Andreas, and now I hate it here.
* Bring back character perks for doing hangouts, that makes it well worth doing them. And make them available more often. It seems like half the time I try to hang out with someone, they say "No, we've got sh*t to do". But yeah, I can think of Franklin having gang member backup and out-of-the-trunk weapons sales from Lamar, Trevor getting maybe the locations of all collectibles on the map from Ron, and Michael... I dunno, getting a discount at Burger Shot from Jimmy? I'm working on it
* This one's minor, but let Michael hang out with Tracey. It's so weird to me that she's the only De Santa you can't hang out with as him, and yeah she'd be limited by the activities he'd be willing to take her to, but no more than Jimmy already is, and I think it'd make for some conversations that develop more of their relationship (Tracey is the only one to contact him when they move out, after all). I guess you could make the case that she doesn't want anything to do with him, but neither does Jimmy, and you can hang out with him. It's just weird to me.