Nagelsmann has successfully implemented his methods at two clubs, Hoffenheim and LeipzigI'd prefer us to get in a manager who has a track record of successfully implementing his methods and style at multiple clubs.
Names like Sarri, Ten Haag, even Nagelsmann concern me to an extent because their successes seem to have largely been with one club. In these cases you're left with the question of whether the success was down to the manager himself or the perfect storm of club infrastructure, players, circumstance and the manager all perfectly aligning.
Now on the face of it, you might look at the proven track record at multiple clubs and point to Mourinho and how that has turned out, but I'm not specifically referring to silverware. Using Potter as an example, he had his success on a small scale in Sweden, has moved to Brighton and seemingly successfully transferred his methods over smoothly.
I don't know much about Sarri, and I'm too lazy to Google, but it's a red flag to me that his main success was his spell at Napoli, and he has then been bombed out of 2 clubs after 1 season despite winning silverware.
TLDR, I would like Potter, see if he can continue his upward trajectory.
I don't think it matters too much if the younger newer generation of managers have only been successful at one or two clubs. You also have to take into account the length of time spent at those clubs because if they have 3 or4 years at those clubs and have dealt with player turnover like Ten Hag has but still maintained a level of success then that is a good sign too