Quote Originally Posted by Bwian
You could have a building 'Dwarf Temple' that would give certain units...say good standard melee troops... when built anywhere. When linked to special resource 'troll slayer' it also builds troll slayers. The special resources are placed in the map regions. This way, you get a generic building that provides certain units when built anywhere, and very special units when built in special places. Gives us maximum flexibility on the build tree without using special buildings for everything.
This sounds like a good compromise indeed. I believe the RTW mod the fourth age: http://www.thefourthage.org/ has something like that. It also takes into account the historical affinity of factions, for instance it is highly unlikely that orcs could be able to recruit dwarf slayers and so on...