Here's the thing, with the way Brown plays guard lineups I think Monk and LaVine would be fine regardless of who starts. You are essentially just simply upgrading Huerter in a deal for LaVine and replacing production from both Barnes and Huerter into a better creator. I read some blurbs that the Bulls are actually looking for picks for LaVine though. If so, I doubt Monte does that unless things get desperate. The thing about LaVine should be a process of condensing players not impacting much right now (Heurter, Barnes, and Davion) into all star level talent if at it's peak. That's the upgrade. Anything more complicated than that gets dicey.
The idea behind adding another star level producer is number 1, Fox doesn't have to carry ALL THE TIME anymore. It takes pressure off of him. Fox and LaVine can account for 50 points out of your offense nightly. Domas at 18, Keegan at 15, and Monk at 15 means those spots around them can be utility guys. At the spot next to Keegan you can go pick up someone like Torrey Craig, Nic Batum, Jae Crowder, etc. this summer and the fact that they don't produce shouldn't hurt much.