It can be quite interesting watch a PSG without Mbappe next season. I'm saying this assuming that he will accept Al-Hilal's offer of course. If he doesn't I think it wouldn't be reasonable at all. I know that he will have a loyalty bonus if he stays for another season here. However it is just nothing beside the amount he can earn at Al-Hilal in only a year. 

I expect Mbappe to take this offer and play for them for one season. After that he can still join Real Madrid side as Al-Hilal also offer a condition like that to him. In this situation PSG will need to make a big signing. Bringing Vlahovic wouldn't be enough at all. I think they should look for a bigger talent such as Osimhen.