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.Although English football is totally different than Spanish. I think English gameplay is faster than Spanish.
It differs much from Spanish football. I wouldn't say that English soccer is faster, but at least it is rougher and more intense. Messi is one for the Tiki-Taka game but certainly not someone who can play his strengths in fighting games. The best example was the lost World Cup final in 2014, it was an intense fighting game and Messi couldn't play his game and that's why he and his team didn't come to fruition at that time. But the discussion has become superfluous anyway after Messi will stay in Barcelona.