From what I understand, the production shift to the HD500x was not intended or planned - it was due to a supply related issue where they had to use a different DSP processor than in the HD500. The footswitch upgrade was probably bundled in with it to make it seem like a worthy upgrade over the original.
And no, AMPLiFi has nothing to do with the HD series. It doesn't have any of the high-end in and outs - AMPLiFi is lacking most of the features the HD line offers, specifically:
-USB to use the device as an ASIO soundcard
-HD amp models (the AMPLiFi uses older amp and FX models)
I would imagine the next generation of HD type POD gear would take several steps up the bit depth and sample rate charts. For example, 24 bit / 192k, like the new PONO music players Neil Young has created.
"The PONO kickstarter campaign raised millions of dollars, and is the third largest Kickstarter campaign in the history of the site. The crowdfunding campaign for Young's Pono Music closed Tuesday, having raised more than $6.2 million. The campaign's goal of $800,000 was blown away less than 24 hours after it launched in March. The PonoPlayer, which will come with 128GB of storage, will sell for $399."
In that regard, the AMPLiFi is it's own world, and as such makes an interesting statement, though it did confuse a bunch of us due to the "reinvention of the amplifier" promotion. The HD and the DT series still stand as the "flagship" of the brand, until they come up with ultra-HD amp modelling.
I'd imagine the shift will likely address some of the other new next-gen hardware that is out there, like the Kemper Profiler. Perhaps Line6 will adventure into the world of $2,000+ POD gear, who knows - they keep their market share around the $500-$700 range, though the M20d, and the StageSource speakers have started to move their line into new price points, and rightfully so - minus the glitches and hiccups, the StageScape / StageSource equipment is really incredible.
Even though, it still lags behind in the realm of bit depth / sample rate - the M20d does multitrack recording and all processing at 24bit/48k, which I believe has been the standard for all the POD gear since the XT series.
So I'd guess they up that a fair amount, continue to upgrade the onboard RAM and processor / DSP power. In particular, they may get into dual and quad core type stuff - the M20d uses pretty high end Burr Brown chipsets, which are no slouch in the digital audio world.
Perhaps the next generation of POD and amps will merge the best features among the DT amp series, the StageSource speakers, and the HD POD amp and fx modelling. For example, they could overhaul the amount of control the user has over amp models - similar to the JTV Workbench, where you can literally create custom virtual guitars, merging various body and pickup models, they could allow amp models to be "custom" created, at a deeper level than currently available.
Perhaps they could create a true stereo DT style tube amp, merging the DT Link features with the AMPLiFi multi-speaker configuration, or add bluetooth and wifi to the POD series.
I think the world of digital modelling has come along way, baby! What makes it exciting and enticing, is that it captures more than just the replication of an amplifier, but the whole chain of events that occurs to get that sound to a recording or front of house mix.