If we move towards a 433 next season (which I think we will) we will need a box-to-box player.
Louis van Gaal has always worked with a midfield 3 consisting of: 1 holding midfielder (Blind, Carrick), 1 creative midfielder (Herrera, Di Maria, Mata), 1 box-to-box (Fellaini, Rooney, Herrera - although he is not happy about these options) player who helps out defensively and offensively. He's said multiple times that he doesn't like to play with only a holding midfielder and 2 attacking or creative ones. He also underestimated the physicality of the PL so he will want a big and strong one.
Strootman was almost guaranteed to come if he played these last couple of months for AS Roma, but it's not looking good for him at the moment. As I know Strootman pre-injury very well he'd have been the perfect signing and van Gaal is a big fan aswell.
BTB players are a very hard to find commodity these days, I don't think there are that many options available. Vidal, Nainggolan, Pogba (not sure if he fits the description), Schneiderlin, Matuidi are all good shouts. Pjanic is a very good midfielder, but no BTB player. LvG talked about balance and he's not going to buy another creative midfielder as we've got plenty and Rooney can probably play there aswell.