It's near impossible to upgrade on Flick. Pep doesn't want to come back and said it repeatedly, Lucho is involved at PSG for years more, Klopp unlikely as well, we need to stay on board with Flick and hope he improves together with the players. He is a very young manager. He has what experience in club football, 4 seasons? Lol.
We are asking of him to be among the elite, most of the others we could get don't even reach this possibility in the first place.
He needs to learn and benefit from Barcelona philosophy also. It could elevate him as manager and help him reach a superior level. Study managers like Pep, Luis Enrique, and take elements from them while integrating some of those elements into his own coaching style.
The two oil teams City and PSG trusted ex-Barca legends and managers to use their billions for a reason.
That said, top tier managers don't grow on trees. They are in very scarce supply, and upgrading on Flick is simply not possible atm.