I expect Nagelsmann, who is fantastic, to be with us for a few years. I don't see it going horribly wrong and I wouldn't want to swap him for anyone.
That doesn't negate the fact that the average managerial spell for us is about two years and a bit and so we are always in need for excellent candidates. Even managers who do brilliant move on at some point. There's cycles.
Ten Hag, due to his style of football, record at Ajax, and past affiliation with the club, is a top candidate to manage us whenever we would need someone. Apart from that, I like him, I also have a soft spot for Arsenal, I wouldn't begrudge Arsenal if they got him. It would also stylistically be a great fit and a potentially great match for both.
The other top clubs, please get someone else.
Utd can have Pochettino for all I care, Juve can have Zidane, or vice versa, Barca can have Xavi, City can have, I don't know, Lopetegui, Luis Enrique, Pirlo, or Tito Vilanova's ghost, thank you very much.