My prediction is a launching water coaster.
Furie mentioned there already exists such a thing in Asia on Facebook, but I don't know of it.
I think Alton getting rid of their flume is ...weird. Like, really weird. And I'd be super surprised if whatever this is isn't water related. I also think a water coaster is non-coastery but also coastery enough to be viable marketing option post-Smiler from multiple angles.
That's a huge area though, and there were solid RMC rumours.
I say launching just because it's weird that Alton haven't really utalised launches that much considering they need their coasters to stay low... And there was a solid launching RMC rumour for Alton, followed by Dollywood doing just that, so yeah.
Those are my current two pennies.
Water. Launching.
I'd also say "family" but since Merlin are utterly incapable of producing anything for a family audience, either by screwing up height restrictions or a desire to make it aesthetically, narratively and thematically terrifying, I'll hold back on that. But it should be a family ride.
And when I say family I don't mean "lame", I mean accessible. It can still be awesome.
ALSO! Just because SW8 was a woodie at one point, doesn't mean it still is. Things change. Usually rumours that filter down through staff are very outdated by the time they make it to enthusiast ears. That doesn't mean they never were true!
I'd be surprised if it's wood just because of the recent safety issues being in the public eye coupled with the evidence we have of Merlin's research into the perception of wood in the past.