I can't see how it would be uphill struggle TBH, the physical size wouldn't change and from my experience it's pretty easy to implement.
It's more about all the variables and logistics involved with developing a new board with a completely new MCU.
I have to think about what it takes to achieve the wanted features, and balance that with how much time/effort/recources/money I will invest in making it happen, plus estimating how many people will jump to buy the board in order to make it worth those investments on my part. Besides that I usually make things that I tend to use myself one way or another. As I mentioned before, going from a few uA to sub uA to me is not as important as other things. Yes maybe shocking to some people, but to me a few years on a small battery running a sub 10uA is more than good enough for my needs and probably most user's needs.
I have to ask myself - can I make a super duper low power Moteino? - Yes - you could actually do it NOW with a
TPL5110 (
35nA sleep) if you really want to. Beats anything in terms of low power, though it will not have the sleep timer accuracy if that's really the wanted feature. But see
TomWS's posts about TPL/sleep, he is very much a promoter of this configuration.
Could I spend hundreds of hours in design, prototyping, education of potential users and unknown future support, only to satisfy a few current interested users? Sure, but I have to worry about many other things in the interim. Look at the Moteino M0 as an example (which is not sub uA but I made it for other reasons and even with the many available web resources - it's quite a challenge for most users), just a simple board that doesn't seem like much but took a lot of resources to make happen.
But does it make sense to make something very new based on a relatively unpopular/unused/unknown MCU? Quite a discussion there.
Most users of my products are Arduino based and they want their code and libraries and what's out there on github to just work or they won't be interested. It's about less than 1% of the users that are savvy and willing to experiment and get deep in C++ to mod and tweak libraries to make them work on new boards.
There is no "official" arduino based on xmegas, (maybe 3rd party boards with limited library support) so right there it will be a support drain to make all the released code compile and work on the new board. Just a few things that came to mind.