It is my understanding that when TSL opens, the entrance to Toy Story Mania will be moved to the opposite side of the building, facing the rest of the new area.
The Pixar Studios area will no longer exist. Meet & Greets will move to TSL, as well as the snack carts. That street will go back to being a backstage area for Cast Members only, which is what it was when the park first opened.
The building that Toy Story Mania is in was originally used for film and TV production, (believe it or not.) In fact if I remember correctly, the Mickey Mouse Club was filmed there in the 1990s.
After filming dropped off this road was opened up for guests, to connect to the streets of America and eliminate a dead-end near the Production Tour. Throughout the years several shows filled the buildings, including Who Wants to Be a Millionaire: Play It! (Where I got on 3 times in one day! And lost at the 16,000 question every time! At least I won 3 hats that day.)
But I digress. Here is a map I made outlining what I believe the be (somewhat) close to the plan for TSL and SWL's layout. Note that the two new lands will probably connect, recreating the non-dead ends that were eliminated by opening Pixar Place street in the first place... just have to wait a few years until those dead ends un-dead themselves.