You should try the King's Bounty series. Actually, I think that the HOMM series was based off of the original King's Bounty game. King's Bounty was remade and updated, and it's quite fun -- and challenging. It's different from HOMM in that you are completing quests in a large world, instead of fighting against rival leaders, and you only get one character that you move around. You recruit from various structures in the world instead of building up a castle. But the combat is the same.