hrmm... Well, like all other attributes, charisma should give you benefits when its high, disadvantages when its low. So, where could we implement this? I think the truth is, keep the implementations where they are now, just make it possible MORE. For instance, when I think charisma, I think of a person you feel open with that you don't even know. So perhaps there can be many times you can extract information, or quest offers from strangers because of your charisma. This should be fairly self explanatory in examples but if anyone really needs one, ask.
In addition, I think the party member idea thats been in the games is good, though they need to give a better reason. Somehow the "You're party is to crowded" When its just me and sulik seems kinda odd. Perhaps it could be something more like "Nah, your not the kinda person I'd liek to be around." But yea, While this stuff already exists in fallout, I think the quest potential (and of course the options of different ways to solve other quests) should be expanded upon further than it existed in the other games.