As of July 1, Mbappe is officially going to be a Real Madrid player.

I'm very excited about him despite there are still nearly 2 months to go for new season to start.
From now on I really don't know who can stop this team in the La Liga. Normally we would say Barcelona as the biggest candidate. However they lack the strength for doing that. Laporta is also aware of the huge threat now but he can't do anything unless Barcelona finds a lot of money from somewhere. Maybe they can make some money by player sales but I don't know whether they intend to give up on any of their decent players.
Frenkie de Jong has been on the rumours to be sold but it is hard to be sure of it. He is a great player who is quite injury-prone at the same time.

I would have named Barcelona as a possible team to compete with Madrid, but after Laporta fired xavi, then revealed flick as a new coach, I changed my opinion and lowered my expectations. Real Madrid has no worthy opponents in La Liga, and they will dominate the league. Again, Barcelona can only rely on its relationship with Nike over a new deal that gives them money, and Laporta cannot overturn La Liga's embargo on his team registering a new player. There is no concrete update concerning Barcelona's progress in negotiations with Nike over a contract renewal.
De jong's rumor has been circulating for several years, but there has been no definitive word on whether he will depart or remain with the club.
After Madrid succeeded in bringing in Mbappe, the gap in La Liga began to appear, because Barcelona and all the other La Liga contestants were unable to build the same squad depth. So it is not surprising that next season Madrid will appear impressive, while Barcelona will face the opposite situation because of the difference in the financial strength of the clubs they have. The dismissal of Xavi and the appointment of Hansil Flick will not necessarily make the situation better, Barcelona will certainly experience ups and downs in performance next season.
Joan Laporta is certainly trying his best to bring Barelona out of the financial crisis, although I assume this will not be easy to make happen. The most immediate challenge is trying to retain key players, Hansi Flick must find a way to convince players like De Jong to stay. If Barelona loses a number of its pillars, then they will be far behind Madrid.