I'd love something like Zach Collins, Blake Wesley, 2 2nds for Brook Lopez. That's probably only possible if the Bucks continue to really struggle, including at the point of attack defensively and want to shake it up. Collins and Portis are redundant, but I think Doc undervalues Brook, and getting to 48 minutes of great rim protection (and, in the same way as CP3 is a point guard mentor, having Brook mentor Vic on all the big man positioning / 2.9ing / Screen setting stuff) would be really valuable.
After that, obviously, shooting is the priority. Hard to build a good offense without people the opposing team is scared of shooting 3s. Eventually, you're going to want some shooters that put the fear of god into opposing defences, though I don't think that's a this year thing.