I'm going to agree they have plans for this plot. They have several other disparate plots floating around the same area, as a PP put up on a map, that I think are less useful and more speculative. But I think this plot is, at this point, aimed at another whole gate and hotel(s) complex. I just don't think it's necessarily imminent. I just think the land came up and it was an opportunity they weren't sure they'd get again to haul in enough land. The 100 acre addition is clearly to provide frontage and access as well as to expand the site.
As for transport between the parks and the new plot, it just depends. For example, it's hard to build the Ministry of Magic on the new plot and have it keep the immersive magic in a connection if that connection involves public stops. It has to be something direct. Even the Knight Bus (compliments to JK Rowling for this nightmare of transportation), which makes stops, would be hard to replicate in terms of magic on a public road. However, if all you need to do is shift people back and forth in a non-immersive manner, a bus would work fine. It's not like Disney's internal bus transportation doesn't stop at lights and stop signs and intersections. But I just don't see the city and Universal really working together to build a direct connection along the city's right of way. Universal would have to connect all those disparate plots and it's probably not worth doing when you can just run buses.
However, I think it would be the greatest middle finger ever if Universal built a PeopleMover type thing to get people back and forth. Just saying...