I heard that SE doesn't want to give us a whole slew of new abilities for the next expansion, because of the clutter. Character builds could totally be a thing for 4.0, the more I think about it. I don't know what type of character builds you're thinking of, but a few ideas I've had about it are:
A: A "talent" system much like in WoW. I think this might be a risky step to take, because it runs the risk of making each job even more cookie-cutter than they already are. People with certain builds would be shunned because their build doesn't work as good as this build, or this build is incomparable to that build.
B: Astral/Umbral Soul System: It sounds a little hokey, I know--I'm talented when it comes to thinking up hokey, wacky, weirdo things--but hear me out. What if we could fuse abilities, skills, and spells to further customize and make our jobs seem more like an art? There'd be a bit more diversity, I think. And the Astral/Umbral thing could be like swapping between two sets, much like changing Job souls.
C: I haven't thought of this yet, but it'll probably be just as wacky as the first two.