10 Party joinable NPCs sounds like too much to do but we'll see, even KotOR really had maybe 5 that were written completely IMO.
I think it's good to have a decent sized group to show some diversity though, even just to show that some characters are more interesting/unique in the game world than others. Not at the expense of fewer well and completely written "main" NPCs though.
Party NPC death would be a bit of a problem I think, playing other games I would usually just reload as a convenience if that happened. I think it could be interesting but it works fine the way it is, it worked okay with BGII where you had 6 party members and morale variables to show some reaction from NPCs but you could also readily ressurect them afterward (if you've won the battle and they didn't get chunked or something). If death was going to be put in however it would be interesting just to see some reaction from party members.
Being able to dump and fight/kill/argue with some of them would be nice too, have some dialogues with a few options which could lead to some fights later perhaps.
Being able to do things like that in the BGs when you met a new NPC was quite entertaining, the scuffles and killing between party members were too, a bit disrupting though