There are a handful of things to change to differentiate builds. An actual level cap is one, but I don't think that will happen, or even think it necessarily should happen.
Playing with the perks and traits is step one towards creating build diversity I think.
Creating actual need for certain SPECIAL like strength, charisma, and intelligence would help as well. Since it is possible to max out a skill even with 1 intelligence, means intelligence and strength are mostly negligible and can be dumped if you want to maximize everything else.
Charisma has historically been a dump stat in fonline since the earliest days, but I do suspect it has some more use than strength and intelligence for builds (mercenaries and such, but haven't followed the meta in that regard closely enough to know how it is. I'm playing with such a build at the moment and still tweaking.)
At its broadest builds are a balancing act of tanking and DPS. DPS can be split into a couple possibilities of criticals versus bursts for the most part. There is a small subset of disable based builds, but these are more of a support style (but with the skill system as it is, its easier for everyone to be a doctor to mend crippled limbs, so it comes down to more of knockouts and armor bypass)
I've always been a huge fan of the potential for buildcraft in fonline games. I've been trying to test things out and see what the meta is these days before I make any explicit suggestions though.