I love playing both the heroes as the overlord but I always feel that the additional weapons and gold to spend, do tend to favour the heroes more then the overlord card deck. True, there are some evil twisting cards in there but...i don't know.
Descent was made to enjoy both as the hero side as the overlord side. If not, they wouldn't have placed in the oveerlord cards in the first place. So there has to be a way to give the overlord more powerful weapons, monsters,...
I think it needs more concentration on lieutenants levelling almost with the four archetypes a pick of races and an xp based system that suited them popping up slowing down the heroes then disappearing imagine a necromancer rocking up raising dead minions into skeletons or zombies then running off as the heroes give chase, but if caught he dies etc start again if he escapes gets better skills etc.
Also campaign cards that the OL could play to say trigger an assassination attempt in the inn etc and cards to change minions so a dragon now breaths lightning or ice much like talismans alignment cards
Just some thoughts.