That's a really interesting question. For me the player either needs to be able to play wide and central am on cm and cam, with a preference for the latter if we sign a single player but absolutely ideally we'd get someone like Schneiderlin + an AM/AMR.
For CAM/AM I would absolutely love Koke, and quietly think he will end up being better than Pogba, but it would take a miracle to get him this year at least so that's doubtful. Speaking of Pogba, he is a possibility although I am wary of changing the entire system just for him. Isco is similarly talented, if more attack oriented and if there is even a whiff of Madrid selling we should bit their hands off. Kovacic is someone I'd also be seriously interested in us looking at, and I feel like if we're looking to add depth and we have the money, that sort of highly promising starlet should more a priority than getting a good deal on someone slightly underrated in their mid to late twenties. Signing someone who plays here would also give Cuadrado the season to come good.
For AMR/AM, if we want worldclass and are willing to pay then it's hard to look past Reus and Griezeman (I want to taste Mike's tears if we sign him

). I would have liked Sterling at 30m but 50 is ******* madness. Felipe Anderson is a really interesting prospect as was Firmino.
These are all personal preferences though, and there are plenty of other brilliant players we could be getting in those positions if we're willing to drop the cash.