Splash contest!
Everyone must use a mono + splash. But that by itself is boring. So restrict the size of the splash. Say, in the first round, you can splash up to 3 or 4 cards. Then, in the middle rounds, a medium splash, ~6 cards. And for the finalists, they must use a splash (a big splash) of 8 cards. Then add a card copy limit to keep things interesting. So, no copy limit for small splash, 3 or 4 limit for medium splash, and 2 or 3 limit for big splash.