Powell would be good if he comes back as a CM. IIRC, he played for most of last season as a #10. He has a better chance of breaking into CM than #10. With Herrera already in, if we secure Vidal, Fellaini will go, unless he gets converted into CB.
If di MAria comes in, ideally, Nani, Young and Valencia should go, since we'll be using di Maria on the right, and Januzaj/Kagawa can be played on the left. Zaha/Mata can be backup on the right, and we have Lingard in reserves, But since Valencia signed a new contract, he'll probably stay, while Nani looks like he's definitely leaving. I dunno if Young will have any takers though.
If Hummels comes, there won't be any departures from CB, but I'm hoping to see Micheal Keane being given more chances. Carrick and Cleverley should be given atleast one more season, and then a decision can be taken. Fletcher may probably leave, although that would be painful for me, he's United through and through. Plus, he was excellent for us when he was completely fit. Sad case of injuries ruining another excellent player. Although, I feel, Fletcher was always underrated by almost everyone..
Hernandez will leave. Another player I really like, but he simply won't get chances and should resurrect his career elsewhere. RvP will probably be upfront, with Welbeck as backup striker (I'm assuming Rooney will #10, although I'd love to see him being given a run as a striker). Henriquez will probably be loaned out again, while James Wilson will be Backup striker.