banther between characters is always apreciated.
not pop culture references tho, they age poorly.
Those should be , if anything, reserved for "rare" voice lines, which mostly are easter eggs.

as for how many, on one hand, less is more, on the other hand i want options.
I think Mask of the Betrayer had a nice sweetspot for companions.
If theres too many then they ofthen end up shallow. I dont realy care about companion quests, they ofthen feel forced and tacked on.
I do however want them to have a lot of dialogue and reactions to choices that you make.
As a rule of thumb, id say have as many companions so you could have two complete parties in total.

Beeing able to level up your party members yourself would also be a big one for me. Or more to the point, a major disappointment if it werent there