I think the next game due out with party NPCs will be Bioware's Star Wars game for Lucasarts, and that probably won't show up until early next year.
But there's a persistence to the Morrowind universe that's kind of nice: if you treat somebody well, word gets around. If you join a group, word quickly gets around to the rest of the group, and its enemies, as well. So you can build up a reputation, for better or worse. I've found that the occasional merchant for whom I've run tasks gets a much higher opinion of me, and that translates over time into lower prices for goods sold by her and higher ones for goods sold be me.