James, a pure transport would be a fantastic idea in my opinion. Don't include any DAC or amp. Look at all the users that have an ALO CDM, Chord Hugo/Mojo, Oppo HA-2, Shure KSE1500, and a few others. These are very popular and I'm sure if there was a user friendly transport for these devices at a reasonable price it would sell well. I imagine around $200 would be good and widely accepted if you get everything done correctly. Battery life should be fairly long I would imagine if not using a DAC or amp (my X5 gets around 30 hours using coaxial output) and a very large storage capacity is at the top of most people's list, especially if it's dual full sized SD slots.
It's a tough call for the operating system as I know there is a significant user base that wants to use streaming apps like Spotify and Tidal. No matter what you choose in the end I recommend you make a transport that outputs bit perfect data digitally with no DSP (except for a quality equalizer).
There are already enough DAPs in the market, including your X series. This would be great just as a transport.