Umm, it should probably be pointed out that Ahab, even if mad as a hatter, had a very careful and perfectly workable plan for catching Moby Dick. He might not have outlined the course of his life beyond the successful kill, but that's quite different from lacking a plan due to madness.
I disagree, perhaps you misinterpreted my statement.
The OP's question is "Let us imagine that Khan wins in TWOK. Then what? What is Khan's plan?"
Khan, like Ahab, had no plan for AFTER they've achieved their ultimate goal. After being exiled on Ceti Alpha V, his entire life's purpose became killing James Kirk. He became so consumed with this desire that I don't believe Khan had any reason to live beyond ensuring that Kirk dies. If Khan had been successful and killed Kirk, I don't believe he had any aspirations as far as what to do next. He wouldn't threaten the galaxy, he wouldn't have any demands, he probably wouldn't even return to Earth.