Now that F&F is almost officially open, I want to throw in my two cents as to what I think is next.
What *should* be next is replacing Supercharged. The Tram needs some help so I hope they are prioritizing putting something in that large building sitting empty.
Once that's done, I have to what wonder what type of standalone ride is next. I won't complain about another dark ride, but the park already has a solid collection of them. I don't think the park is desperate for another one.
What I do think the park should focus on is another coaster of some sort. I understand all of the challenges and the noise complaints, believe me...but I don't think a single ride has garnered this much buzz at USH like F&F. Even anecdotally a lot of my friends are bugging me about when and "how" they can ride it. You can also see it with the guest feedback of Mario Kart when it opened when the ride was said to be "too slow". People want something fast and thrilling and people are responding. That's why I think the park will probably look into (And probably WILL) look into putting another coaster in the park.
That being said, it for sure won't be another big intense outdoor coaster. The ride sounds like it was a logistical nightmare to clear, but they got it done. I think that's the only major thrill coaster the park will get. With the major thrill itch being scratched, what they should have is an intermediate family thrill coaster....something well-themed, but a good in-between of the kiddie coaster that is Hippogriff and the crazy thrills of F&F. Something like Hagrid's would be amazing as they can use the current Tram loading plaza for such a ride. It likely wouldn't be Hagrid's but at least something of a similar calibur.
I would also like to see the Mummy get replaced with a better indoor coaster.