Now we can only add a number of units, it'd make scenarios (on a historical map especially) far cooler if you could for example unlock a new unit after a set number of turns. Another possibility would be to open more of the so far unplayable map and set it to neutral or to set various prerequisites, ie if someone has capped this, he gets a new unit or unlocks area or gets set number of units. I'm sure others will have more ideas.