I guess this is where I'll bring up the elephant in the room. A lot of the shorthand for these concepts in D&D is the alignment system; in other games your actions fed into this abstraction of personal morality principles and philosophy, in better games it could also feed into how your character behaves. I think alignment should be in D&D but I like if it's kept under the hood, if people aren't told what their alignment is they can't min-max it as easily, and you can end up with a character who's alignment and behavior are the result of roleplaying instead of the other way around.
As for general demeanor? I'd honestly love it if we could get like... "Confident", "Kind", or "Cruel" tags that determine our character's expressions and general body language. Playing an evil Githyanki was awkward when he struck a Superman pose and smiled every time he met a new person.
I like the tag system from, I think it was in Pillars?, where you gained traits through repeatedly acting certain ways, I respect the attempt to incorporate a way to develop your character's personality this way; I wish more extensive systems like that were developed.
I also think that something terribly lacking in every RPG is how asymmetrical and idiosyncratic interpersonal relationships can be, but that's another topic all together. SaurianDruid's comment about Githyanki made me think of this, a Githyanki should behave very differently to other Gith than to anyone else, something most personality systems don't handle well.
