Well he pretty much has to go out with a bang-probably literally. I cannot think of any solution that has him leave and the others not go find. Michonne,Daryl,Carol, etc. simply wouldn't quit looking. Also no way does he just slide off into the sunset leaving the others behind. On a different but related note I think Rick Grimes will finally reach the end of the road in the book. Not based on anything other than a feeling he isn't needed anymore though the introduction of a new character being called "Governor" is an ominous development.
It will have to be ther biggest and grandest exit ever, and just think that right now the shown literally and officially has it jumping the shark moment, as will not be interested in watching anymore...
The Gunslinger in the Fear the Walking dead to me shpuld come over and become his replacement!