Priority:
1. Left-back
2. Midfield
3- Right-back
4. Right-wing
5. Center-back.
I think the left-back position is the most important one to get a high-quality player to take because that is where our best offensive players currently are good at playing, and they need the support & freedom a top quality left-back can grant them. After that I'd like to see us not be too reliant on Matic & Pogba to stay injury-free for us to be bale to string two passes together or create anything at all.
Right-back & right-wing would be great to have higher qualities on, but we could make due if we properly fixed the midfield options and gave our left-sided attackers a better partner.
Center-back would be awesome and the icing on the cake. I think it's needed to push us playing further up, but we have players to counter in the squad currently, so It's not the end of the world if we go in with our current bunch and play lower for another year as long as we adress it the year after..
Options:
LB: Alex Sandro (David Alaba)
MF: Sergej-Milinkovic-Savic (Mateo Kovacic & Ruben Neves)
RB: Thomas Meunier (Djibril Sidibe, Sime Vrsaljko)
RW: Riyad Mahrez (Gareth Bale or move Sanchez to the right)
CB: Varane (Alderweireld)
Preferences are based on reputation and/or minimal viewing for a lot of them.
I'd prefer to sign 2 midfielders. One to help Pogba out with the creating job and one to rotate with Matic for the defensive job needed.
Best case scenario for me being a 11 and bench when all fit like this:
Lukaku
Sanchez ----- Mahrez
Pogba - SMS
Matic
Sandro - Varane - Bailly - Meunier
De Gea
Bench: Romero (GK), Smalling (CB), Young (LB/LW)Valencia (RB/RW), Neves (DM/MF), Herrera (DM/MF) & one of Martial (LW/ST) or Rashford (LW/ST).
Other squad-fillers: Lingard, Jones/Rojo, Tuanzebe, Pereira, Lindelöf, Mata & TFM.
I'd happily see the back of: Rojo/Jones (one not both), Blind, Darmian & Shaw. I assume Fellaini will be signing for Milan or someone else.
Next summer I'd consider upgrading on Bailly if he isn't made himself a starter throughout the season, I'd push Lindelöf & maybe the other of Rojo/Jones out of the club to make way for a new first-eleven CB & Tuanzebe to be given the squad space. Effectively making Smalling - Bailly our second-string CB choices.