Midna could come back if you think about it, because Zant broke the Mirror of Twilight into Shards. But he still managed to get to and from the Twilight Realm, but its not explained or established how he does it. If he can do it, Midna surely can being as shes Twilight Princess. So there could be some more story there, was Zant dead at the end of the last game? Maybe, maybe not. I cant think of a plot off off the top of my head though. Maybe when Midna broke the Mirror, it unbalanced light and shadow, causing some concequence? Who knows...

What would be cool, I think, is something called the Battle of the Triforce or something, with a massive battle taking place at Hyrule Castle. Midna bringing her people, Zelda bringing Hyrulian soldiers, and Link there too, defending Hyrule Castle from Ganon or Zant or someone, would be good, with Zelda and Midna fighting too, all around Hyrule Castle, maybe shooting arrows and bombs down at the enemy and trying to stop them from entering the Castle. Would be a good climax, anyway, just an idea