I would support no respawns. Lords of the Realm style ticket scaling. Say you have 10k vs 5k battle. Maximum amount of players is 120 right? 10k/5k is 2. The team with 10k troops get 80 players, the team with 5k troops get 40 players. If the team with 10k wins and loses 30 of their players, they lose 3750 troops on the world map.
10 minutes of battle and you're done. If you blow it you lose, no chance to revise tactics. One chance to defeat the enemy. This is very hardcore and it probably wont appeal to most. I'm just putting it out there that this is a possiblity too.
Or make it really like Lords of the Realm and give each troop rescaled hp to make him represent the amount of tickets he costs. With 15k troops divided over 120 players every player gets 125 times their usual hp. If you have 55 hp normally, you would get 6875 hitpoints in this case. You run out of that and you're done.
You might think that is ridiculous, it is really, could work though.