Not much of plan beyond vengeance against one man really... the Genesis Device can only be used once. Then he's still facing overwhelming forces intent on hunting him down.
Khan's a super smart guy with an entire community of super smart followers. I'm sure he'd find a way to reverse engineer the Genesis device and manufacture super weapons from it.
I've wondered whether that was his intention all along. After all, it's a foregone conclusion that he was aware of the destructive potential of the device and the position of power such a device would put him in.
Why waste a colossal bargaining chip on one planet, when you can have many?
Although just the Reliant, wouldn't be enough to protect him from the rest of Starfleet once that's done. He'd need allies.
Plenty of other genetically engineered species in the galaxy. That, and along with the power of Genesis as a motivator, he'd have a lot of allies in no time at all.