To be able to choose whether to go first or let the game go first
Add a 2 player option through which two people can play in one single device.(not online)
Add an AI which makes decisions based on your moves
Make a settings page where people can choose difficulty, and make 3 AIs which make easy, medium and hard moves respectively.
On the settings page you can allow people to choose white or black theme as well.(I know most people use black but still, the more features the better.)
Lastly I know it's a lot of suggestions so take your time and have fun while doing it.