I totally agree, i think you shouldnt be able to build anything.
It would make larger armies have a bigger advantage as you say, but it would also make fortresses in fortress maps a much bigger advantage, which i think would add a lot, you would have to gather a larger army to besiege a fortress map.
I think war of the ring should be just battles, you spawn with an army and then use it to its full potential and not multitasking in the base, if you want to play like that, it should be in normal skirmishes. Ive always liked the fact that you spawn with your army and then its just about how you use it to win the battle, the fact that you can build stuff just defeats the purpose.
Plus ive always liked having battle about troops, i sometimes with my friends build up armies and then battle and the first player who reaches 0 command points loses, and then just quit the game^^
Bonus:
Also if this would be implemented in the future, i dont know if this would be too hard to do, but remove all the creep lairs on the map, you just want to battle your opponent, the creep lairs purpose is to stall the game, but in this mode you already have an army and cant improve it in any way so the they are just completely unnecessary.