As for defenders I agree that Donck looks promising in that role and he is only likely to improve with age as he bulks up a bit. Unless someone of serious quality - in the Dias bracket, though I don't know too much about him personally - I would be very content to keep going with what we've got.
If so, perhaps even if we bring in another defender, we would need a cm with some creative skills to compete with Mou (good luck!) and as back up. Sander Berge from Genk would be quality - big guy with a good pass who can also move the ball forward very well. Apparently he turned down Brighton and perhaps Sheff Utd in the summer, but we should be more attractive.
Another forward wouldn't do any harm either, but that would mean fewer chances for Cutrone and Neto - unsure if we really need another one up front.