Depends on what formation we employ going forward to be honest. At the end of last season, most of us were rather convinced that we would see more of a 4-3-3 look under Van Gaal, and here we are now, playing an iteration of 4-2-3-1.
With the latter formation in mind, we need :
A right winger - This is an absolute must for the club. Mata is fine and all, but he's just not a winger who can challenge his marker on a consistent basis. We need someone who can inject some much needed incision, dynamism, and pace to the attack. Bale would make a lot of sense but Perez is infatuated with him. Reus is the next best option, and has some experience with playing on the right hand side, but he's right footed and Van Gaal has a fetish for having left footers on the right flank. Those are two standout options, apart from maybe Felipe Anderson and the likes.
A proper #10/ support striker - Yes, Ander played there for Club Athletic, but his best position is still as the attacking box to box in a 3 man midfield. Rooney might be a good option, but he can be really inconsistent, and we do need someone who has the touch and creativity to be the brain of our attack, a bit like Blind for the defense, and Carrick/ Schweinsteiger for midfield. Van Gaal had Litmanen at Ajax, and Muller at Bayern, and we honestly don't have that caliber of #10 at the club right now. Someone like an Isco would be brilliant, his work-rate improved a lot under Ancelotti and he can be our equivalent of Silva. Ditto Griezmann, who has really blossomed at Atletico albeit more as a support striker. Even Mario Gotze would be worth a punt, all the talent didn't just desert him. That said, Isco and Gotze might be more difficult to extract from their clubs than a Griezmann who has a set buyout clause.
Centerback - Blind, to his credit has done really well alongside Smalling but the sample size is infinitesimal, so we can't really gauge the long term viability. Again, the options will vary depending on Van Gaal's level of insistence on having a left footed defender man the LCB position. Which might rule out Daniele Rugani, and John Stones, both excellent choices, the former probably being the best central defensive prospect Italy has produced in a long time, maybe even better than Chiellini on talent alone. Which leaves Laporte, who has been slightly inconsistent for a few months now, but is still a world class defender in the offing.
Apart from those three positions, the squad looks quite decent. The key to the team's performances will be fielding player at positions that complement their playing style, instead of rejigging the line-up every week to cover up for deficiencies. Then, they will start gelling, and get some chemistry going; it's really easy to forget that most of the players under Fergie and Moyes are now gone, we have a lot of fresh blood playing in Van Gaal's system for the first time, and it takes time to build a team from individual pieces. Might need better quality in terms of depth though.