Maybe it could be something like Red Force at Ferrari Land, if an Intamin launch coaster is rumoured? The only spanner in the works there, though, is that BGW apparently don't work with Intamin after issues they encountered with Escape from Pompeii, which is apparently why Mach Tower is a Moser and Verbolten is a Zierer. So maybe an S&S compressed air launch coaster, like the Bullet Coasters at Happy Valley parks, but 315ft tall, would be a good fit? I've heard that the company's Chinese launch coasters are supposed to be brilliant, and I would also imagine that it would cost a lot less than a ride from a company like Intamin, which is good for a company like SeaWorld, who aren't in the best of financial states at the moment from what I can tell.
Or maybe this might not be a launch coaster at all, as BGW already has 2; both of which are relatively modern. In that case, a giga would be an obvious option, in my opinion. A B&M would be nice, but I don't know if it would be a bit too costly for SeaWorld. BGW apparently don't work with Intamin and I'm not sure if Morgan would build them a giga coaster anymore, so that's all the current manufacturers who have built giga coasters ruled out. On a random note, does Morgan even still exist as a company?
So maybe we'll see a giga from a company who's never built one before. My personal top two contenders for this would be Mack and Vekoma. Now let me explain why. Mack Rides have built 3 hypercoasters, and I could imagine that this hardware is easily transferable to a giga coaster. Vekoma have not yet built a hyper coaster, but they did propose a hyper to Energylandia, so they're clearly interested in building a ride of that scale. Also, a giga at BGW could potentially be the ride that Vekoma needs to accelerate itself back onto the US coaster scene. They've proven that they can build a decent coaster with rides like Lech Coaster, so I can imagine that a Vekoma giga coaster would be quite well received. I'd imagine Vekoma are also a fair bit cheaper than the likes of B&M, Intamin and Mack, so it would be a decent, but also budget-friendly gateway into the world of giga coasters for BGW, in my opinion.
I'm not necessarily ruling out B&M or Intamin, though. SeaWorld built Mako for SeaWorld Orlando in 2016, so the budget might well be there for a B&M giga. Intamin have built many rides for other SeaWorld parks, and Pompeii opened nearly 10 years ago (I think?), so it wouldn't surprise me if BGW were willing to reconcile with Intamin.
P.S. Sorry for long post. Also, sorry if any of the ideas I've suggested are a little too outlandish.