Messi in very end of his career does not want to gamble with a strange coach whom he has not yet worked with before. So from this point of view, Pep is the best coach for Messi. Pep won't leave Manchester City so Messi will come to join the party with Pep.
I doubt that Messi can really help Manchester City. He is 33 and English football is different from the one he used to know in Spain. The game system would have to be adapted to him, which means that ManCity would have to change its current system. That would be bad for a well-rehearsed team. Besides, it would take at least a season to make it work. By then Messi will be 34, almost 35, and none of this will work.
Pep is always tinkering with his squad and I am sure he already has everything figured out in his head. If Messi goes to City there will be adjustments from both parties but I expect them to click and produce results almost immediately. I am thinking Pep would play Messi as a false 9, while at Barca he destroyed Real in one game with Messi playing just that position. City has world class players on the flanks and this kind of tactics would suit them perfectly. If Aguero isn't fit striker position is a bit weak, Jesus never really shined there so this is another point for false 9 theory.
Only problem is Messi defensive running, he has been outputting seriously low numbers in this category, worse and worse every year. That could possibly prove a problem for City.