First of all, it's been a long time since I was last in here. I recently dug out my two Gigabeats which had Rockbox on them and was surprised to see the batteries would still take a charge. I was also dismayed to see that my eyes can no longer read the tiny font on the screen, I'll have to figure out how to install something larger. (sigh)
When I was last hanging around this site there was quite a bit of activity, now it seems that the latest release was some two years ago (recent by Debian standards :-) ). At one time there was some thought of designing a custom media player for a rockbox target. I guess that went nowhere. However, "maker" development boards have come a long way since then. Arduino like clones with Cortex M4 ARM processors running 120MHz (overclockable to 200MHz) are available, along with an i2S amplifier module (IE: external D/A with headphone driver), and TFT LCD displays with SD card socket, all from Adafruit. The SAMD51 is available with 512K-1meg flash, and 192-256k ram, I wonder if that is enough. The Teensy 3.6 is similar.
I also have a few older smartphones in my drawer that are no longer in use (Moto X gen 1, BlackBerry Priv), I wonder if they can be jail broken and used as general purpose devices (though this might require careful surgery to open them up and somehow connect a JTAG, plus the cost of a JLink-Edu).
Don't be afraid to call me 'stupid' on these ideas, I'm sure I don't know what I don't know here, despite being an embedded software engineer for several decades. (Mobile devices are NOT my strong point).