But also in order to make this unit more effective in battle, the Treasure/Supply Transport WILL ONLY BE SPAWNABLE through a special unit and the summon will be diferent for each race. But as any other spawnable, it would vanish after 2 or 3 turns.
Here my sugestions:
Alladin - for humans (Jinn/genius/genie/gênio???... Dunno how the lamp guy is called on english so Alladin for short)
A guy that flies on a magic carpet and make your wishes come true (or no)
he could spawn it from a set amount of time! (every 3 or 4 rounds) or by sacrifice one allied unit (still thinking wich could fit)...
Treasure goblin - for orcs
For each kill made by this goblin, it spawns a transport
Elf alchemist - for elves
He can transform through alchemy the dead corpses in gold! (one treasure transport for each 2 or 3 corpses, but instant cooldown)
Golden Skeleton - for undead
He can revive corpses as undead transport - something like zombie reaper, but instead of zombie, he spawns the transport (2~3 rounds cooldown)
As always, leave what do you think about