I really wish I had more of the know how to make a Kickstarter/Crowd Supply for this. I imagine an mp3 player with native Rockbox support would be very popular, especially in the form factor of an iPod and/or a Sansa Clip.
The hardware that ran early Clips, IPod shuffles, and other small players existed 15 years ago. A dozen Chinese manufacturers still sell small players, which now are about the only option left, but they skimp on quality components like batteries and decent software.
Sadly, I work a daytime job and don't really have the time or will to actually capitalize on this. I do think that'd be a great way for Rockbox to fund themselves though. Make a Kickstarter with Rockbox officially supporting it, and the profits could go to the upkeep of rockbox.
But like you said, they're geared towards either the high end like FiiO or the very very lowend (i.e. third world countries where smartphones are only beginning to become ubiquitous in the last few years).
I just want a simple self contained mp3 player. If mine breaks, I think my next one will be an iPod loaded with Rockbox.
Power is hardly relevant in this day and age, as any device with a decent firmware should be able to handle a ton of files and formats (this wasn't always the case, hence the need for rockbox).
Smart phones have versatility, but you sacrifice a lot on the music side. The audio hardware within the smartphone isn't as good, and it took a lot of years until there was an android player that had the bare minimum of features required by an audio enthusiast, namely lossless support (flac), replay gain, and gapless playback. Additionally, there are now players that run android and have all the "fruit", so the benefits are dimished.
But don't overlook the versatility of a standalone player, because relatively cheap devices like the Fiio X3 II that I am still running have other advantages compared to phone. It supports big sd cards (wasn't always a given on smart phones), has long battery life, has physical buttons (so can be operated without removing from pocket), and also functions as a USB DAC. So, lots of wins there.
I got overwhelmed at the prospect of starting the CAD schematic and layout for the project (I've done simple microcontroller boards in Eagle, but never one this complex). The daunting task was wiring up all the data and address lines for 2x large RAM chips and 2x Flash chips, and my ADHD kicked in, plus frustration at not being able to use programmer abstractions to do circuit CAD. I also kept not finding the time to port Rockbox on the hardware development kit.
Side note: there's giant hole in the market offerings for hobbyist single board computers. You have microcontrollers with KB's of RAM, and things like the Raspberry Pi with GB's of RAM, but there's nothing with ~16 MB of RAM and a low power 32-bit processor. The chips exist, just no one has created a cheap & compact hobbyist board (i.e. not a hw dev kit).
ST's STM32 Discovery boards [1] fit that bill reasonably well. They're inexpensive (many under $20, most under $100), supported by an open toolchain (GCC / GDB / OpenOCD), and provide a wide variety of peripherals. Most parts don't have more than 1 or 2 MB of on-chip SRAM, but a decent number of their boards have 8 or 16 MB of external SDRAM.
[1]: https://www.st.com/en/evaluation-tools/stm32-discovery-kits....
They seem to be in the business of making Linux style things, and I wonder if this would be up their alley.
I use a CLip Sport for workouts, but they are not nearly as good as the Clip+