Any Mass Effect fans around?
We need to go to Mars first and find the Prothean data cache with some Element Zero. Then eventually get to Charon and have it thaw to uncover the Mass Relay.

Oh yes, according to Wikipedia:



So the next logical name would be Hypersonic, and not Transonic. However, since we want a Zone which ProblemSolver can reach, Hypersonic is out as well because it corresponds to a minimal speed of around 6000 km/h.
The next zone would have to be Mach 2, but then again I doubt ProblemSolver can even reach that, so unfortunately if we want to remain with the standard we will have to keep Supersonic as the last named Zone, simply because any further realistic objective is humanely impossible to reach (And therefore useless to implement.)