@SuperKali - That is a very interesting concept - a Hero Beacon. I do like it. It's like a Tavern, but mana is the currency instead of gold. Maybe the Hero Beacon is a building that's powered by mana (instead of gold upkeep). Heck, maybe the Hero Bacon is even a summoned building so it doesn't chew up time in the build queue.
Some factions could have a Hero Beacon (or whatever it might be called for that race) and some might have a Tavern type building that does need gold to summon heroes.
This hero summoning building would obviously need to be restricted to the Fortress/Capital City.
Then we might still have an actual Summon Champion spell (for each circle maybe too) which specifically calls a magical type of being (kind of like summoning a demon with a pentagram).