Simeone and Ancelotti are the obvious choices if they became available, as the next best options after Pep, in terms of their achievements.
On one hand I'm not sure if Simeone is a good fit for United ATM. He's achieved incredible stuff so far at Atletico, but I think United fans want champagne football with a bit of joy to be the aim. Simeone is an attritional coach based on his career so far, a little like Mourinho, in that he likes teams to win a battle rather than mesmerize their opponents. Of course he is so young so will hopefully add many more bows to his repertoire with more experience, but if we go for him he'll have to show that he can do that quickly. Incredible start to his career though. His title win is possibly the most impressive I've seen from a manager in a top league.
Ancelotti on the other hand is proven at top clubs that play more expansive football and is undeniably someone who seems to bring people together. He's been loved pretty much everywhere he's gone which suggests his man management is impeccable. The downside with him is that I'm not sure he's a squad builder. His challenge would be to turn us into something we would not be.