Just more 4D roller coasters. The tech has been out there for going on 20 years now, and gives a fantastic ride. No reason S&S need have all the fun. (And, let's be honest, wouldn't they be far superior in B&M or other's hands?)
This. But it got me thinking, though... who would want to build one?
B&M is very conservative, and it doesn't sound quite like their thing to introduce a coaster train with so many moving parts. Not to mention, the specially manufactured track.
Intamin could try their hand at it, but they've been burned numerous times on trying to innovate and ending up with reliability issues. Nobody ever bought a second Wing Walker from them, and building another one with a more complex track and train... I can see why they wouldn't try to develop it, and why parks wouldn't buy it.
Vekoma ...maybe. They're good at doing unconventional stuff. But they don't tend to build coasters on such large scales as the 4D coasters require. Large trains and special track requires large curve radii, which again beget very big coasters. And the biggest full-circuit coasters Vekoma ever built was
this defunct thing in South Korea, followed by the two at Fantasy Island. Vekoma don't really market themselves at that scale.
Mack? Possible, but it's a little bit outside their comfort zone too. What with the extremely wide vehicles and special track and stuff. Then again, Time Traveler has proven they can be unconventional, so perhaps one shouldn't count them out entirely.
As for S&S, the old adage holds true: Once bitten, twice shy. The 4D coaster spectacle sunk Arrow, and it hasn't been a very successful coaster line since then either. S&S has the demonstrated capability to build them, but I can see why they would rather offer something else to their customers instead. Coasters that offer the same level of thrill while being smaller, cheaper, more reliable and less likely to bankrupt somebody.
I don't really see any other manufacturers willing to take on such complex and huge coasters either. Never mind that parks aren't very likely to buy them anyway. We really don't see as many coasters taller than 50 meters with inversions anymore, whereas the standard 4D layout towers a whooping 65 meters or taller.