The blue pipe services a sink, and I do believe continues on to become a vent above the sink. The red pipe services a toilet, and the green services a washer and another bathroom group on the 2nd floor.
As you can see, there is quite a bit of maneuvering going on up at the top of this stack to get everything connected, and then routed down to the stack. I would like to reconfigure this drain arrangement to take up less space, to minimize the amount of boxing in I will have to do when framing this out. I'd like to run the red pipe vertically down, to connect with the top of that 45 degree offset, and run the blue pipe around to the top of the wye where the green is connecting. This is crudely illustrated below.
I would use a long sweep 90 to transition the red pipe downways, and a wye to tie it in. I can't find anything in my plumbing codes to prevent my doing this (IRC 2015 and IPC 2015), and the inspector I asked would only say "Yep, that's allowable." Will running the red pipe down in to the middle of that 45 degree section cause me any issues with future performance of these drains?