To be frank, speculating about any new hardware is largely academic. There's nothing from both Castle or Advantage Six to firmly indicate new machines are on the way. It simply remains a (rather remote in my opinion) possibility.
I think the software side, i.e. RISC OS itself, deserves much more attention in my opinion. The A9home's version of RISC OS needs to be completed. It's taking too long and a completed and refined OS is crucial to appreciating the machine fully. Personally, I think speculating about hardware is reminiscent of (self-built) PC's. RISC OS machines are quite different in that there's a very specific combination of hardware and OS. Like on a Mac, the software is the computer. RISC OS is the computer and the whole reason why people pay a little bit extra for the experience.
In other words, RISC OS needs all the attention it can get and I think a good place to start is asking when the current A9home gets 'its better half' completed, so to speak.