I think he should stay in Milan because he has achieved something with this team at this stage of his career and I believe it should be memorable for him. But we all know and agree that ibrahimović is a different type of character. So, I am not going to be surprised if he decides to play for any other team. It is going to be interesting to see when he is going to retire considering that he is really old right now.
But given the age he is at and the good things he has achieved this season it would be wiser if he hangs up his boots and moves on to something other than a player.
Zlatan's role is indeed very good for Milan but there is one thing he has to think about which is not necessarily next season he will reach this achievement and indeed now is actually the right time for him to retire as a player.