I agree, I like M92. So far, it is the best 10" e-ink reader. It is well usable now. The weakest point is not hardware but firmware. Firmware is gradualy improved, so I do not see any problem in buying now. (With current design cycles of electronic devices, sooner or later there will be a new model for sure...)
Maybe more RAM would be helpfull in some cases. And higher processor speed never makes device worse.
Back to original question. As far as I know, no succesor of M92 was officially annonced. There were only rumours on one Russian forum. Booxtor denied those rumours and Artatech man confirmed them.