I don't think Pep Guardiola will manage Chelsea if he leaves Manchester City. Of course it doesn't make sense that he got everything with Manchester City but chose to leave City to join Chelsea. If you pay attention to Pep's coaching history in two different leagues, of course his next step will be to leave that league. He left Barcelona and did not coach another team in Laliga and also he left Bayern Munich and did not coach another team in the Bundesliga and it is very likely that if he left Manchester City he would not coach another team in the Premier League.
Pep Guardiola really appreciates the fans and he doesn't want to betray up on coaching another team in the same league. Yes, it is very likely that he will go to another league after his contract end with Manchester City or he will stay with Manchester City for a long time. Many say that he wants to return to Barcelona someday and it could happen.
Regarding with Pep Guardiola history becoming manager, never manage two teams in the same league such as after leaving Barcelona he move to Bayern Munich before becoming Manchester City manager. I don't think with next challenge if leaves Manchester City he will be Chelsea manager. But its can't guarantee if Pep Guardiola leaves Manchester City won't to be Chelsea manager depend on how management have planning and good road map such as with Manchester City road map planning.
Pep Guardiola is most wanted manager for all teams who interested to sign him, but its not easy depend on Guardiola is most expensive payment as manager and he won't to be small teams manager who have lower budget for transferring players. I am expected one day later Pep Guardiola will be Manchester United manager looking for United have waiting longer time difficult for winning domestic league tittle.
A manager like him is not going to want to manage a club that is in big trouble, to be honest. He can go to a club that has a good financial situation and try to improve the squad and try to make them the first team on the table. But he is probably not going to want to go through the trouble of building a club from the ground up. He is a good manager he can do that. But sometimes it is just not worth it. The success is getting right now is going to be compared with the next club that he’s going to manage no matter what is the difference between those clubs. So why would a manager want to make himself look bad?