A keyboard in turn has limitations over game controllers. Why not provide a game controller interface for your tablet instead so that manufacturers can provide arbitrary controllers instead of just flat ones the same size and shape as the tablet?

iOS 7 for better or worse is doing that instead:

http://www.ign.com/articles/2013/09/19/gamecase-the-first-co...

http://news.cnet.com/8301-13579_3-57589649-37/ios-7-ready-ga...