The land between USF & MIB will likely not be used for quite some time still. Creative is based back there and there's also the HHN tents and just general back of house stuff. Now with the new land, this all can be moved, but that's over time.
As to building something next to Fallon... that's not really all that likely unless HRRR bites the dust (coaster supports are in the way) and Universal is comfortable with losing Sound Stage 33 for a ride. If they did decide to use the plot though, there's plenty of space back there to make it work if they were to go right up against the backstage road like F&F and (to an extent) Mummy does. There would just have to be some shifting of backstage infrastructure, but again, with Site B, this is made possible as a location like SS 33 could be built over there. 33 is a nice space to hold event though, very convenient to USF and is used A LOT.
I just think both are longterm expansions. KidZone, Fear Factor and Shrek are more immediate. Then by the late 2020's, all of Springfield will be on the chopping block... and probably MIB and Mummy sometime in the middle of all of that. All of that practically gets you into the 2030's without using a single *expansion* pad in the park (not replacing).
Oh and what about Monster's Cafe? Tear that thing down and build a ride using that land and the Donkey M&G area.