If its intended to keep just a single attack type (melee), perhaps have different classes that have a different special effect. ie:
Rogue = First hit is a sneak attack, dealing extra damage. After their foe is dead, they stealth, and aren't able to be hit until they attack again. Also they boost loot gain whenever gold or items drops.
Warrior = They force the strongest enemies to prioritize them in a fight. So in a boss fight, the boss will always target the strongest warrior in the group. They also get more defense and dodge per level.
Barbarian = Can occasionally rage out, attacking twice in a turn, but reducing their dodge ability to 0. They also attack slightly faster in general.