Actually no when I asked I hadn't even made that obvious connection. I think main reason to avoid that is Wii motes had wrist slings, no doubt if you implemented phone gyroscopes it'd end with a social media post of someones damaged phone and tv.

Reflecting on my time playing Wii the sports games were ok but nothing screams 'gotta have this' to me.

If I could give one unsolicited piece of inspiration it'd be to watch this episode of Community: https://www.youtube.com/watch?v=fOueERJzjrU

There is definitely an awesome couch multiplayer game in there somewhere