A reliable centre back who can become a mainstay of the first team and if van Gaal wants either a left-sided centre back or right-sided centre back (as it is a slight idiosyncrasy of his) then I hope it's the former. Smalling can be excellent, if he maintains his fitness, while Jones and McNair can provide competition, but we only have Rojo for the left side and Evans who I think we should sell. There's a sparcity of quality, experienced left-footed centre backs, so someone like Laporte might be worth a shot. I know I'm in the minority, but we only need one new CB IMO.
A versatile fullback primarily used in a right fullback capacity. Valencia has been very solid and I've always felt he can have his place here in some capacity due to the work-rate, strength and pace he provides even if he has been one-dimensional and void of confidence for a few seasons, but I wouldn't feel comfortable relying on him for large spells of the season at right back and that definitely seems the case with Rafa's injury woes. We need a capable challenger for his position and someone who can cover for Shaw also. Clyne perhaps?
In midfield, I still feel we are lacking some steel and someone who can contribute in both thirds of the pitch. We already have two box-to-box midfielders in Herrera and Fellaini, but the former feels like a b2b verging on #10 due to his flair and creativity in the final third and even though he is a good tackler, energetic and generally competent defensively, he isn't a specialist there. As for Fellaini, I only see him as a viable option if we are using him in a utility capacity or to negate certain opponents.`Someone like Strootman or Schneiderlin would be a great addition to our midfield, providing more balance and making it very complete.
A pacy, goalscoring wide player is crucial too. Take di Maria out of the team and we are largely deprived of any pace in the team, save for the fullbacks. It's a position we certainly need another in, even if we do continue with narrow, winger-less systems. It's offers variation and a different alternative to sustain some intensity if we are chasing a game. Currently, we only have Januzaj and Lingard who I would deem suitable for a wide position (ADM has to play centrally) and Lingard isn't a youth player with a guaranteed future here. His prospects are starting to look bleaker. Signing someone like Depay would give further impetus upfront and provide a threat from a different area of the pitch.
A striker, I feel, is dependant on Falcao's fitness and form. If from now, until the end of the season, he can remain injury-free and show signs of the Falcao of old, he might be worth signing permanently and that would render the need for another striker superfluous. With Rooney, Falcao and van Persie approaching, or in, their thirties, it should allow for a more progressive transition for Wilson to stake a claim in the first team over the next few years.
Those signings and I would expect a concerted title-challenge right until the very end next season.
...so i'll just leave this to van gaal to think this about.