Manchin would simply switch to the GOP. This is not unusual. Senator Shelby from Alabama did it long ago; Jeffords switched to the Democrats. Manchin switching to the GOP would assure he’d be senator for as long as he wanted to.
Manchin has also been mentioned for Energy
Manchin lost a lot of popularity here after his gun control statements. The ones he made with Toomey after Sandy Hook. I doubt he could get elected to anything again.