Don't think it will be a "soon". My wild guess would be earliest around 2019 (which would be nice, since then I have the money to buy an updated version immediately
).
Have read here in the not so distant past that the AX8 is considered a "not so old unit".
Seeing from own experience (working in a company which develops own measurement equipment), development and product support it's a time consuming thing. Sure, it certainly has a distinct taste to upgrade all hardware to a new platform, thus maintaining only one codebase - but my wild guess is that this is "flagship first, keeping the older stuff run awhile". And as far as I see it, they need at least two compilers for Axe-FX II and AX8 (and certainly some special platform dependent code), so three platforms appear to be manageable.
I got excited about the FX III, but - this doesn't mean Axe-FX II will out get no updates of nowhere (and so AX8), because that appears also completely in contrary to the claim "Future-Proof Technology". If I remember correctly in the Wiki, it's stated somewhere that Cliff wanted to support the Ultra, but there was no memory left to support it further.
But a better display for the AX8 would be nice, for sure.
So: I don't see a successor popping up out of nowhere in the next months - but if I am mistaken, then I would be happy, of course