Interesting idea. I haven't heard any rumours or anything from Sonnen or his camp but It's definately logical. Rampage is chomping at the bit to leave the company so theres no way he'll be fighting again after his next one. Sonnen could always float around the LHW division but of course he won't have the same success at LHW as he did at MW. Okami, Stann and a debatable decision against Bisping are his best wins. At LHW he will come up against Gustaf, Rashad, Phil Davis, Hendo, Machida etc. Include JBJ and Daniel Comier in that list and thats 7 fighters straight away I see as having a good chance against, if not beating, Chael. I like Sonnen but if he doesn't retire like you mentioned I can see him doing a Rich Frankiln, being #2 at MW but barley in top 10 at LHW.
Bisping fight wasn't debatable, Chael clearly won in my eyes and even so Marquardt was a much better win than Bisping and Stann, he was ranked number 2 at the time and everyone thought he'd steam roll Chael and get another fight with Anderson but Chael completely put it on him and that's when he emerged as a star.