Make it so if you choose to play on BLU team, you must have the intention and be willing to go warden. Very simple. Having freedays over and over again where REDs massacre BLUs gets stale very fast. It's a very simple addition.
Also, if it does happen where BLUs just don't want to go warden, add a warden randomizer. Simple.
Also, if it does happen where BLUs just don't want to go warden, add a warden randomizer. Simple.