As I mentioned a few months ago, it's being set aside for "Expansion" but may only be accessible through the DK area. That coupled with the fact that the early pitch artwork showed a DK play area that is not being built with phase 1 has led me to the conclusion that that will be the expansion. You know, since it's a skinny and oddly shaped area only accessible through DK courtyard.
And it's not unusual for Universal to add more stuff for kids after opening the main area.. Not sure if it's comparable but it wasn't until IOA was open for a little while that they added Flying Unicorn and Storm Force. The Simpsons Ride opened well before Twirl & Hurl, etc etc.
Again, just my guess based on the info I've heard.
Oh, and that little building outside of the permitted work area on the right is for Security and break rooms I believe, and should be staying. The expansion pad would be around it. It's the white area in this image below (and may not include the little bit bumped up against ET as that would need to remain an access path for Team Members.)