I think Thorpe would be the next to get one, on that undeveloped island in the back of the park. Maybe they'd even fit two coasters on those islands, and one behind Saw. After that, the question of how many big coasters they would want to operate would begin to arise. At some point, the park will hit the point where it has to take something down for every thing that goes up. In Thorpe's case, I could see little happening until Colossus finally croaks in the mid-late 2020s, at the ripe old age of 25-ish.
I don't know Alton Towers that well (in truth I don't know Thorpe very well either, but anyway), so that's harder to tell. If there's still room in their park/development plan/operations budget to keep adding new coasters, they probably will, and presumably it would involve something big at some point. If not, Runaway Mine Train would reach replacement age some time soon, but mid-size rides like that can be kept going for a long, long time, and it probably wouldn't be replaced by a big coaster anyway. Likewise, Nemesis and Oblivion will start to show their age sooner or later, but they are beloved enough for the park to keep maintaining them, maybe to the point of total refurbishment, and their unique ground works means it would be prohibitively expensive/impractical to put up anything else in their place anyway. The first big coaster I could see go from Alton Towers would be Rita, probably to be replaced by another launch coaster of a more recent model (we're talking along the lines of Helix and Taron... or beyond, since both of those will be a decade old or more by then).
As for Blackpool Pleasure Beach, I think that will be the only realistic third candidate for the foreseeable future. And that park is pretty crammed already. Some of it with rides they can't replace, even. And it's not like they can build in the car parks, because of proximity to residential areas. Those car parks already serve as buffer zones. They will manage to squeeze in Icon, but I can't see how they'd be able to fit in anything classified as a "BIG coaster", even if some older coaster was to get the wrecking ball. Maybe when Infusion is one day scrapped, in which case I could see them getting a Dive Machine or something similarly tall yet compact.
Flamingoland is a wild card here, though. It's a rather large non-Merlin park which might find courage/money to dabble a little in the thrill market. Hard to tell what they'd build, though. Perhaps they'd get a special offer from a new actor in the market who'd like to get a reference project.
