Khan left Earth in disgust of normal humans. Why would he come back? I imagine he would try and conquer other worlds. Reliant would be his flagship/pirateship. (Until he ditched it when it was less useful, like he had no problem doing with the Botany Bay.)
Khan was run off of Earth because they (the supermen) lost. He had a score to settle with Earth and likely would've went out in a blaze of stupidity.
You mean Khan is the kind of guy to hold a grudge?