Someone like Frank De Boer who shares elements of Van Gaal's core philosophy would be the perfect replacement to be fair. If Giggs is found to have requisite traits in 2017 then perfect. If not then a quick rundown of De Boer's resume viz-a-vi the requirements of a United manager would suggest some good pedigree :
Winner ? Yes. Won 4 straight league titles at Ajax since taking over as the manager. Also some high profile victories vs European clubs with stronger teams and bigger budgets - 1-1 and 2-0 vs Milan 2010 that finished 1st in Serie A and had Thiago, Seedorf, Ibrahimovic, Nesta; 2-2 and 3-1 vs City 2012 that finished 2nd in the league and had Aguero, Nasri, Toure, Kompany; 2-1 vs Barcelona 2013 that had Neymar, Iniesta, Pedro, Xavi.
Tactically proficient ? Yes. Can play a lot of different formations to suit his team and has outmaneuvered some very strong teams and managers.
Can he promote youngsters ? Is there a bigger football school than Ajax ? De Boer's current first team is almost entirely filled with Jong Ajax graduates. Every year bigger clubs steal his biggest players - Mido, Suarez, Stekelenberg, van der Wiel, Vertonghen, de Jong, Alderweireld, Eriksen, Blind. But De Boer just fortifies the team with youth players. And each year they get stronger and stronger.
Character ? Very good. Captain at Ajax, Barcelona and the Dutch national team where he also the record appearance holder before Van der Sar. Natural leader.
European experience ? Yes. Has played in the Champion's League in each of his 4 seasons so far and one well considering the players at his disposal.
Can he handle big name players ? This is perhaps the only big grey area. Managing players of Ajax Amsterdam and the likes of Rooney, van Persie, Mata, Falcao, Di Maria is an entirely different proposition. But De Boer is well respected in the game, was Liverpool's first choice before Rodgers, Guardiola wanted to make him assistant at Barcelona instead of Tito and Frank was himself one of the best players in the world. So there's no reason to doubt his ability to get the respect of the current lot.
Long term ability ? He's only 44 and will be 47 when Van Gaal's current contract expires. Could go on to manage United for 10-15-20 years.
Apart from him Guardiola will be out of contract in 2016 and will most likely takes another sabbatical so he might be free in 2017. However Pep won't stick around for more than 3-4 seasons which is a big negative. Klopp is another one who might leave Dortmund. That said I'd rather he stayed there and continue his body of work TBF, adds so much romance to the game. Mauricio Pochettino and Roberto Martinez are good candidates from within the league.