Nobody knows when Ferguson will call it a day....I don't even think he truly knows. Personally I think Pep Guardiola going to Bayern tells us a couple of things....he wants a stable base to build from (Chelsea will never offer him that), and he is keeping his options open for a potential move to England. Becoming Manchester United manager must be incredibly appealing to him as, like Bayern, there is that incredibly stable base offered by Ferguson's tenure.
Both have great youth facilities, infrastructure and scouting networks, a winning mentality plus huge financial resources to tap into. United are keenly following the Bayern club model by installing club figures in prominent positions and this is sure to help any incumbent manager settle with the support they require. Cutting his teeth at Bayern will give him a taste for a different culture, a different club structure and release some of the intense pressure he endured at Barcelona.
From United's POV he must be attractive in the sense that he does not court controversy, is humble and modest with his achievements, shares a passion for developing young players and is a visionary coach and mentor. Simply following Ferguson may not be enough, the new manager will need to supplement the core base with his own ideas, helping the club evolve and not become stuck in the transition between past & present.
He would be my personal preference....by quite a margin.