We could make markets an alternative upgrade to inns.
Or include an option to change schools to fully loaded turrets for
a moderate cost. So that you can defend when needed, but pay a very
high price for it.
We also could make unit upgrades exlusive.
For example impose (Swim Level) + (Attack Level) <= 4 for warriors.
Or make explores upgradable to super strong marine units (fish) which
can't walk on land - as an alternative to air attacking explorers.
How about unit fusion: 2 warriors come in 1 super warrior comes out.
Maybe Attack Level 3 should require that.