For the record, I too think Vekoma belongs with "the big guys" already, if for nothing else then by virtue of having designed and offered coasters that cross the threshold and then some. A couple of them are even under construction right now. Also, thanks @Indy for correcting me on the Gravitycraft thing.
Among the ones remaining, I think Golden Horse is the most likely to be the next ones to reveal indigenous designs of something really big. I don't quite see Maurer or Zamperla taking the step up so soon, and even if Beijing Shiabolai does, I'm not wholly convinced the design will be indigenous. Golden Horse seems to be more liable to design something of their own instead of emulating other manufacturers' designs, as that dive machine prototype appears to demonstrate.
I also believe GCI or Martin & Vleminckx might build something that's a couple percent faster than their current biggest creations, technically taking the step up from ~98 km/h to ~103 km/h and technically crossing the threshold, but they won't break any paradigms and create the next Son of Beast or anything like that. Business as usual, plus a few percent, not adding much spectacle.