I think Milan is already well-covered in midfield (after the arrival of Kaká) and goalkeeper. What Milan need is a striker to replace Inzaghi and a Defender to replace Maldini. In My Opinion, Milan need a striker with height and strength but not a slow one. Juventus have Trezequet, Man Utd have Nistelrooy, Internazionale have Vieri, Leeds have Viduka, Lazio have Corradi, Barcelona have Kluivert, Bayern Munich have Santa Cruz. When Milan had Bierhoff (even if he's slow), Milan won the Scudetto. So, a striker like that is needed. A true striker, not a striker who's waiting for mistakes from the linesman. The best in the world I think should be Vieri or Trezeguet. A defender that would suit Milan would be Charles Puyol of Barcelona. Everyone knows his quality. No doubt about that. If Milan think that he's too expensive, Milan could buy Bonera at least. Maldini is sure irreplaceable and was(or is still) no.1 defender in the world. Puyol is young so he has a lot of time to improve his ability, especially if he practices with great defenders, such as Maldini and Nesta.
I hope one of these transfers would go through before the season starts.