I got this idea while watching a YouTube retrospective of the Castlevania series. It allows you to play as 2 characters, based on the gameplay feature in Castlevania 3. You select your two characters at the beginning of the game and can only be those two. A button would allow you to switch between them. Like Castlevania 3, you would share health, meaning in SMBC, both characters would either have a Mushroom or not, regardless. This would be a very fun mode similar to Survival, in my opinion.