New genre
Incorporate twitch ?
Separate players, not so much like P1 (Mario) and P2 (Luigi) with same experience
Reward break / handle fairness of hand off
Player not playing gets benefits from player who is player. Big brother type mode lets younger sibling "keep up"
Different type of narrative design. Allow one player's decisions to help or hinder other person. Punch in the arm -- why'd you do that!
https://blog.twitch.tv/twitch-for-game-developers-b664163bed65
Leapfrogging concept. By waiting for your turn you learn the path or technique to take, but are also gaining some power to assist in your turn.
Stealing lives concept
Can become an entire development genre as well: games could be unity-only, adhering to specific requirements = GAME API. So Players could connect to a central (curated) server that had these GAPIs plugged in. On each death, the game would change. Makes it interesting / fair if someone is like super good at one genre (street fighter), on loss, game would switch to another genre (tekken) so the second player could have revenge. Fairness is parity of time. Time is the only value of measure that all humans understand. If you can play for as long or just a little bit longer than your equal partner, then it feels fair. If you hog the controller, other player stops caring as much and it stops being the same game ecosystem. LIFEFORCE -> MARIO --- also nullifies the immediate effect of P2 knowing why P1 dies on that level. GIves them chance to make the same mistake.