Just tossing this one idea I had.
Add Hero altar building that is limited to just 1 per region. If it's too difficult to make that restriction on command card they can already be placed on map as other reclaimable buildings.
Each altar can turn one villager into hero of different class. Assigned hero can't turn back into villager or sent to limbo. If hero dies he can be revived using essence (1000 for example).
Next I'd like to suggest adding Special Boss regions where a really powerful boss monster live. This Boss should be so powerful that you won't stand a chance to defeat him with golems and turrets (or won't have enough time to get enough of them). But at this region you will have power to summon all heroes form every region to fight the Boss - the more regions you have the better are your chances. If hero dies here he also dies in his origin region and needs to be revived. Defeating Boss can unlock new regions, larger buildings or even finish the game.
Don't know how much different hero classes there can be but there should be at least fighter, ranger, healer, and/or mage, summoner.