I found V's story to be solid and enjoyable overall. Maybe it wasn't perfect, but I'd much rather have such a story that lends itself to stellar missions rather than have a serious and solemn story that results in minimalistic gameplay. IV took more of the latter approach, and while the game was good overall, it lacked the absurd fun in the gameplay for which the series had previously been renowned. I felt V struck a good balance of not too serious and not stupidly ridiculous either.
If they choose to take a more solemn tone again with the next GTA in which miniguns and double-ended dildos are to be cast aside once again, then I'd personally like to see the gameplay reinforced with more RPG elements that serve to pull me into the story. For example, if I'm given a list of exotic cars to boost, I should need to develop some skills, like lock picking, computer hacking, etc. to do so rather than merely getting in and driving off. I'm not saying the series should turn into a full-blown RPG, but looking at what makes games like Fallout 3/NV, Deus Ex HR, TES, etc. so insanely addictive, it wouldn't hurt to steal a page or two from games like that.
Anyways, I think they can continue to write great storylines. The humor in V is still true to GTA form and that's the most important thing. Even with IV's more serious tone, the humor was still on point.