I don't think they have to overhaul or add additional systems to Sims personalities. They just need to strengthen the current ones. A hot-head, for example, should default to more mean interactions with other Sims, be angry or tense more often, etc. A lazy Sim should have a much difficult time keeping their energy bar up when actively doing things, but should also gain energy more quickly when sleeping or even just sitting down, unlike other Sims. Etc, etc. Just tweak the current system to be more obvious.
I feel like the differences are already there, it's just that they're more subtle than they should be.
