If he would like to quit "properly", he should quit in 1-2 years, leaving Mercedes. It will be very tempting to race "one more last season" but then the most probably it would be much weaker team and he would regularly finish in the second half. I do not think Red Bull or Ferrari would take him, so he will end in any other team, like in happened to Kimi. But going to Williams... would be funny.
I think Lewis will be here for the next 3-4 years. Why ? Simply because he won't let Alonso compete longer than him. It's his main rival and he always will be not Vettel or Rosberg or Max. Will he go to weaker car ? I doubt it but I do see him trying to make a comeback for McLaren or if Toto leaves for Aston Martin , I could see him go there as well. We are heading towards exciting years of racing in F1 when teams and drivers are chancing pilots or engine suppliers very quick.