There's two features I would love to see added to the server's remote control API. Of course it would require changes from all client developers as well, but I think it would be worth it.
1. Audio/subtitle stream selection. I know MBC can't capitalize on this, but every other client should be able to play specific streams rather than the default. Especially if transcoding.
2. Add the Id of the client that initiated the last received remote control command in the SessionInfo. The reason I'd like to see this, is for when the mobile clients exit and relaunch, it would be nice to poll the server for currently playing media and if the initiating client ID matches the mobile client ID, expose a now-playing widget in the client. Same concept as what Android is doing with Chromecast when you relaunch the client.