I'd like some units that grow in power with age, and sometimes with a disadvantage too this. Like a vampire lord, slowly eating your population but growing in power about one upgrade every 4th turn or so perhaps...
Kind of like the eater of the dead, but less expensive, and more useful I guess
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And maybe giving mages a chance to increase their magical abilities when they naturally age(dont want any basic mages getting hit with decay and turning into gods here )?