I don't see G-Force coming back and sadly that's a real hit to their thrill attractions. If they could I'd want them to get a new coaster but as of now I doubt they can afford anything substantial to the park. As much as I'd also like to see an immersive attraction (Haunting and Pirates Adventure were hits) I doubt they can afford the maintenance for an attraction like that. Despite losing G-force, their collection of thrilling flat rides is really impressive, I'd honestly say second to Thorpe Park.
The only two thrilling attractions that I think could offer something different that the park doesn't have is a Gerstlauer Sky Fly or a Skyline Skywarp model. A Sky Fly is a cool experience however the GP may compare it to Air Race and with Pleasure Beach not too far away it might not be seen as unique. There isn't really anything like a Skywarp in the UK, it's got a small footprint and honestly if they marketed it as a roller coaster I wouldn't be too bothered. They could go with the classic or horizon footprint as a first in Europe or if they invest more into it, an Eclipse model, first in the world and a great replacement for G-force.
If they'd rather stick with family/kids, which they most likely will, then it'll probably be another Thomasland expansion :T