I'm not sure if any topics like this have ever been created before, but if so give me a link please. If not, here I go.
My suggestion is to implement creatable npcs that will be with you until they die. Make them cost a healthy amount of cash (25,000-30,000 caps maybe). When you buy these npcs, (either from a new npc specifically for this, or a mercenary recruiter with added dialog) make them able to be customized with the same character creation screen that a player would get, choosing traits, s.p.e.c.i.a.l, and tags. Starting at level 1, obviously, as well as having the same amount of experience needed that a player would. Also allowing the owner to allocate the skill points and perks to his/her desire. Possibly customizable name, age and gender as well? I've been working with fallout 2 and the cheat boy mod, and I've found that implementing this is not very difficult. (at least in F2) Not sure about an online server sided game, although I don't see where that would make much of a difference. With this, I've gotten similar results in game-play to fallout tactics. I control both myself, and my companion during combat. I haven't been able to test this in real-time combat due to fallout being only turn-based. If any developers are interested in implementing this, or info on how I did it, pm me. (I'm sure we can figure out a much easier way to do this then my method)
Let me know some opinions on this.