Suggestions/requests for NPCs:

Option to have NPCs treat one another as solid, and halt, turning in a walk pattern, if they try to collide.
Collision between NPC(npc), NPC(object), NPC(player), etc., generates event (e.g. planned encounter, special dialogue, and so forth).
NPCs take action on collision, with similar collision options (e.g. steal from player).

I presume some of these can be handled with scripts, but I don;t know how much you want to force the user to rely on scripts to do these types of effects.