Well, a Halo mobile game has already happened; it's called Halo: Spartan Assault and it was released on Windows Phone. Because Microsoft owns the Halo license, it's extremely unlikely that the game (and any upcoming Windows Phone Halo games) will ever make it onto any of the other mobile operating systems. If you want a Halo mobile game that badly then I'd suggest that you'd get a Windows Phone next time you need a new phone. Simple as that.