great idea gorgos.
why this would be an excellent idea:
1. creates more alliance team play
2. All races would be able to trade their units for others for more variety
but I think they'd have to be roughly at the players level example as follows:
A new player to the game could not be traded a lvl 20 infantry unit.
What could be implemented I think is:
you can trade units of the same level Example:
level 20 swordsmen for level berserkers
level 5 rangers for level 5 crossbowmen
etc you get the jist of things