I don't think there will be a Hogwarts Express train connecting the two parks for a number of reason:
1. Having to check everyone's ticket to make sure they had a 2 park, park-to-park ticket would be a huge headache, and really hold up the lines.
2. Construction and operation of a train that runs backstage would be a logistical nightmare.
3. There really is no room to do this backstage.
4. There would no longer be a Macy's parade because of lack of space and clearance for balloons and floats.
5. Mardi Gras may have to be ended for the same reasons.
I do think there will be an expansion of Potter, most likely in Amity/Jaws, but who knows, perhaps it will be LC and Amity is going to be something entirely different. I think Marvel will be here for a while, or else Universal would not have spent the money to update Spiderman. However, Marvel could lend itself very well to a LOTR theme-"Escape from Mt Doom" coaster, "Two Towers" free fall ride, and a mines of Moria or something like it in Spidey's place.
Hey, I could be wrong, I just can't see a train happening, and I am keeping my fingers crossed for something LOTR related sometime, I hope I hope I hope.