I actually was thinking about a system like this and trying to figure out ways to keep it from relying too hard on the RNG. I made a script back for VX Ace that basically had this system but some of the features I wanted to add were beyond my abilities.
Some of the ideas I had:
Individual characters (or classes, but I don't think this would be compatible with a class system) could have innate potentials. Let's say each stat has its own EXP gauge that fills up when you win battles or complete actions. Depending on a character's potential some stats could take longer to fill than others. Actions taken can also give EXP to their respective stats (weapon types determine actions as well; in the SaGa series all physical abilities increase HP, all swords increase strength and defense, guns increase dexterity and agility, magic spells magic and MP and so on). Some special enemies might even give an extra bonus to magic attack EXP or something. Since defeating enemies could give EXP along with action EXP (which would be more like a bonus) it lowers the need to grind endlessly or drag battles on forever to appease the RNG gods.
So to throw out an example, a giant human wall character is naturally suited for tanking while the crafty nimble one would be better at dexterity-oriented battle styles like short knives or guns, but if the player really wanted to they could break it somewhat by altering stat gains (by forcing characters to use a lot of skills or equips they aren't very proficient with).
But it can be frustrating if you want a team that can all use magic, but only have 1 or 2 characters that have a knack for magic. So there should be a way to mitigate that deficiency, since the system is all about customization. You could do that through equipment, possibly, or some other gameplay feature like items or a sidequest. Give your natural-born fighter some low level magical equipment that increases magic attack/defense EXP gain until they're proficient enough to use the advanced magic equips. Specialized/advanced equipment could be locked by stat levels so you can't give your fighter end-game level mage equipment right away because they don't know how to use it.
Alternatively, if an equipment system is too complicated/not effective enough, go to some witch doctor in the woods who will give you innate potential-altering items for odd favors, or do some weird ceremony that lets you rearrange some of your character's potential points (if it's a more modern/SF setting it could be a quack doctor or mad scientist
).
I think a system like that would be fun to play with, but I also really love Romancing SaGa and Frontier. (I'm in the camp that hated FF2 though. the mechanics were way too broken)
