I would love to see a world 2 where monters are stroger, more life, more damage. Some of then could summon stronger monsters, or have 1st hit invencibility, or fly! they would be imune to melee damage.
Foes that attack from behind your formation, wizards whose attacks ignore all the armor or invencibility, a troll or something that use a club to do a splash hit. A poison monter, a poisoned big monster.