Opinion and suggestions on balance and heroes:
After playing around: the XP changes introduced in the latest patch- challenging has become a cakewalk for me difficulty-wise. (I always play on challenging because I hate cheating AI's one way or the other, and I also believe it's the difficulty the game is intended to be played at)
A +damage/level weapon is almost an instant win button right now. I'd suggest those items need to be nerfed.
I think weaker heroes make for a tougher game. Right now it's too easy to get a steamroller. It also devalues the late-game hero recruiting techs.
Also, I think the game needs to spawn some random heroes over time , as too many of them are killed quickly.
My suggestion : starting on every turn after x, give a y% chance of spawning a hero on a random unclaimed tile. Maybe adjust location based on who is losing.
Also, prestige should increase the cost of heroes, as another balancing mechanic.