My argument for a likely retheme versus a rebuild is due to the nexus of F&F’s location and the importance of varied experiences—while looking at narrative options for retro IPs. Because, here’s the challenge:
The PERFECT ride system for BttF is a Scoop… but Transformers is right there. You wouldn’t want two scoops side by side. So, fine… indoor coaster with launches and screens. But then you’re right between Mummy and Gringotts. That would put THREE indoor coasters in a row. You could put a Kuku in there, but that’s still a somewhat similar experience to a Scoop and the bench seating doesn’t really match a Delorean. A slow dark ride would get the same complaints Mario Kart gets. A straight simulator (flying theater or otherwise) will immediately get compared to the original. So… a retheme of FnF to BttF provides different experience from everything around it while allowing a different experience from the original.
Jaws is a different problem. The original Jaws ride took up the entirety of Gringotts AND DIAGON ALLEY, which basically means it was almost twice the size of the FnF building. So, a “boat ride” is unlikely. Shanghai’s pirates’ system could work. However, a water ride also requires the ability to regularly drain. Logically, that’s the lagoon. However, with the fountain show, it’s possible that raising the water level too high can damage the show’s infrastructure (see, the hurricane). So, if that’s the cause, an indoor water ride is less likely. Universal also had a building already condemned from mold because of indoor water use (Poseidon’s Fury)… so I can’t imagine they’re jonesing for an indoor boat ride without the ability for regular drainage.
(If you look at the Dark Universe expansion plot, they have two retention ponds within 300 meters likely developed for overflow. That would allow drainage for a boat ride there.)
A Jaws variation of the FnF system limits drainage and mold concerns while, once again, allowing an experience viscerally different from what’s around it.
I, personally, don’t care either way. I’d be happy if no retro IPs come back but I’m fine if they do. I’m just looking at from a holistic, “park management” perspective.