Fun Gamemode with different ults?

Was trying to come up with an idea for a cool game mode that Riot should come out with next. My idea was that the map plays out on the original Summoner's Rift. Each player picks the champion they desire, however the twist is they get to choose a second champion to essentially use their ultimate spell instead of their own. Another thing to add is that the opposing team would not be able to see or know what the others ultimate is until they end up using it. What do you guys think?