With Mbappé leaving, PSG will undoubtedly find someone to replace him. Napoli striker Victor Osimhen is a potential candidate to replace Mbappe. With impressive performances, Osimhen is interested in many big teams. PSG, Chelsea, and Arsenal are all rumored to want him. With its financial potential, PSG can completely win this deal, regardless of the ambitions from Chelsea or Arsenal.
Actually, PSG still has some alternative strikers, they have Randal Kolo Muani and Gonçalo Ramos. If they don't sign a new striker, I think it is no problem as long as they have those 2 strikers. But it is very possible that PSG tries to sign a new striker because they want to make their attacking line be stronger. They are never satisfied with their existing strikers, moreover if Mbappe left the club.Osimhen is a good striker to replace Mbappe in PSG. However, it depends on Osimhen where he wants to play in the next season. If he wants to move to a more competitive league, he must prefer to move to EPL. But if he wants to get a higher salary and popularity, he may choose joining PSG. In PSG he must be the main striker and be a new iconic player there.
I have heard De Bruyne has some interest in him from Saudi Arabia. Manchester City are said to have set a price tag worth 100 million pounds for him if it is really true though. 
I don't think De Bruyne will be interested in the offer of Saudi clubs. He has a great season with Man City, he has a big role in Man City squad. There is no reason to leave Man City in the near future. I'm sure there is no Saudi clubs that has the ability to sign him for the next season.