I would be shocked if it happened, but hey i would like to see it happen also. More likely is he stays on for this and next season before becoming next man united manager.
The next manchester united manager will be pep guardiola or ryan giggs. Van gaal will retire after his contract ends and have no plan to continue coaching. Pep guardiola has not sign any contract extension yet with munich so it is almost sure that he will move to MU next season
Regarding Pep Guardiola it depends on how it will go with Bayern Munchen this year.
If they win the Champions Leaugue it will be strange if he leaves the team.
Nothing is strange with that, do you remember that he leave barcelona when the team is winning title as well? He may do it once again and that is not strange. The strange thing is he is not signing contract extension until now which means he wants to look for a new team to handle and that maybe MU