In the excellent Let's Play called Syvishtar's Journal, an interesting self-imposed rule is followed:
If an NPC in the party dies during play, then after you resurrect them, you leave them to "rest" for a while and pick up a different NPC to replace them.
How about a tweak mod to apply a 1-week stat drain penalty to NPCs that have been resurrected, to encourage this kind of NPC-rotation-after-a-death play-style? The benefit is it gives a reason to play with more NPCs that you might not otherwise include in your party.
Other ideas for tweaks that could encourage this kind of play style? Instead of a stat drain, it could be that NPCs that you haven't played with for a while get some kind of bonus "excitement"/"restlessness"/"adventure-seeking" effect that gives an advantage (like extra movement speed without needing boots...) but wears off after a while and only comes back if you leave them out of party in an inn for a while.