IMHO, Mbappe is not as good as Dembélé. He is very fast going forward, but he has not faced top notch defenses. In contrast, Ousmane Dembélé is very fast, he has an excellent burst of speed (pique,) and has excellent dribbling abilities. He can go forward, stop and go back, or go to either side, if he wants to. In other words, he is excellent at stop-and-go. In addition, he plays in Germany and not France. The defenders and defenses in Germany are so much better.
Ousmane Dembélé is a very good signing and I believe he is the closest player we can get to Neymar. The other one is Coutinho. We need to replace Neymar, now, and Iniesta next year.
Another pressing need is to get an area offensive player that is a "9." A player with height for headers and that has the ability to push the ball in the net. Alcácer is a major disappointment and contributes nothing to Barça.
I'm waiting for the signing of Ousmane Dembélé to be made official. Until then...?