If we're planning on sticking with a 4-2-3-1 for the medium term future, Saúl and Goretzka would be the top options from the younger lot - because they can both do a bit of everything - defend, press, carry the ball forward, and score (sporadically) - plus they have a bit of size about them unlike a lot of other options who are a bit unsuited to the Premier League. Martínez + Schwiensteiger was arguably the best midfield duo of the last sort of half-decade. Saúl + Goretzka would represent a very shrewd piece of business from that perspective, because the duo could develop into something approaching Martínez + Schwiensteiger's class - given their individual skillsets, which are very similar to the former Bayern duo.
Overall, our midfield is not as strong as it appears at first glance (especially for a double pivot). We need more numbers on top of quality:
Schneiderlin still has a lot to prove, moreso in terms of passing and intelligence on the ball. Might not be apparent against Premier League opponents (a lot of whom sport dross in midfield), but if/ when we go up against the stronger European teams, that particular weakness will be exploited unless we play him in a midfield 3 as a water carrier of sorts where he is instructed to always release the ball quickly to better midfield passers.
Carrick is almost done.
Schweinsteiger is still rather good, but Van Gaal's system stifles him (he thrived more under Heyneckes), he should ideally be rotated very frequently to preserve his fitness, and is removed from his pomp as a midfield general - though you do see flashes from time to time. Will progressively get worse, so ideally we should not build around him beyond the duration of his contract.
Ander's best position is in a midfield 3 where he has the license to roam and improvise.
Saúl + Goretzka would cost maybe €50 million combined, and we'll have an exceptional base once they hit their stride. Plus, despite his decline, Fußballgott is an exceptional leader and mentor; he could be someone they could tailor their games around, which is a good thing given his stature as a modern midfield great. Would add Neves to the list if we can't acquire one of those two, he too has exceptional potential. Though ideally, we would sign someone like Gündogan - who can start contributing from the get go (or atleast once he gets used to the league). Though maybe the ship has sailed with that one.