If it hasn't been suggested yet: Perhaps some different spcies of goblin to expand on hero strategies. And maybe a few hero specific units. Or, bailing the strategy side of it, you could introduce loot and let heroes have equipment to boost thier stats, unit stats, etc. This is a VERY solid foundation to make an epic game out of. 5/5 and more if I could :)
FOR REAL with the undead AND the trolls locked, I might as well go find another game. Serious dislike for locking races on ANY site. Just make them all available everywhere and THEN you'll see the views shoot up net-wide
AMAZING GAME!!! But like everyone before said, too short. Needs some replay or maybe the ability to generate a random enemy army? Also if you start adding stuff, consider a barracks to store a few spare units in.
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It didnt harm WL:CtoA which had the same races locked.