Thanks for the reply. We were hoping there were a lot of options, but Internet search doesn't turn up much either.
if i remember rightly Rubicon's Combat Monsters is cross platform. It's quite deep and strategic, indeed too deep for me but it was great from the little I played when it came out.
World of Tanks blitz is cross platform though I've heard the game having bad experience with android player due to framerate. Some devices are doing worse than the 4s which hovers around 20~35 fps. The last update did speeds things up so anything with a Snapdragon 600+ (and similar) should be getting relatively decent framerate if not better than the 4s. It seem Snapdragon 800+ (and similar) are getting 60 fps. The best devices for playing this game is anything running an A7+ since its hitting fairly high framerate and are the only devices running on ultra resolution.
All of the "Legends" MMOs are cross platform, from what I remember. (Arcane Legends, Star Legends, Dark Legends, and Pocket Legends)