I think he should venture out and prove that he has the ability to manage United because there's no way I can see an internal appointment being made.
The board have seen the current position we are in and won't risk giving it to him now.
If the next manager does poorly, the board will again look for someone with a name to steady the ship and this will continue until the right 'big name' manager finally gets shit right.
Which ever manager does get us back on track (which is likely to be Maureen), when that manager leaves, they wont want to bring in Giggs as they will be too scared to make a risky appointment after all that has gone on. So they will sign another big name manager.
I can't see how Giggs could or even should get the job. Yes Guardiola has done wonders when brought up through the ranks but to be honest, he had the world's best player in his side, which was already filled with superstars, in a league where even if they played shit, there aren't enough quality teams to knock them out of the top 4. It's completely different in the EPL.