You know, one of the little things I like is dungeons where your NPC allies fight alongside you. For example, the Ghimlyt Dark and The Heroes' Gauntlet. I wouldn't mind seeing that happen more often.

As for mechanic wise, I would like to see more non-boss mobs with mechanics. For example, in Amaurot, I think, there's a mob that will always target your healer, and is completely immune to tank provoke and stuff like that. I get a little tired of the mobs between bosses being just small monsters that chase you and hit you and occasionally do an AoE attack.