This seems simple: have every team create their own 30-card 'default' deck, which must get posted in full view of everyone at the start of the War. Then, allow them to use that deck in the place of their normal Vault cards any time they choose.
That way, there's always the ability to play a deck, but you risk your opponents easily guessing what you're going to play and building a nasty counter to it.
Great idea, just make sure to give it some stricter deck requirements so you can't use it in place of an actual similar deck into your vault. One of the staples like Fire immoltion rush that a fair amount of people could predict anyways.