Have two (or more) clients control one controller?
Nvidia Gamestream Co-op has a feature where you can have the client use the same controller input as the host (as in, both would be player 1, and both are capable of sending input to the player 1 controller). This is useful for helping players out in single-player games, as well as providing a way to play turn-based games co-op that don't normally support it, or simply just allow you to take turns playing the game as if you were passing the controller over. It currently seems like, right now, each player has their own controller and there is no way around this. I don't know if this feature is already planned, but it would be a nice addition (I'd like to take turns with my friend playing DBFZ online, for example; currently in order to do so, we have to both disconnect and whoever wants to be player 1 has to connect first). Even if this isn't possible, an easy way to swap controllers would help as well.
Edit: I was wrong about PS4 Share Play, you can't control player 1 if someone else is, but you can 'lend' them the controller, which you currently cannot do without disconnecting right now.