Basic: .ogg / .opus support? MIDI player?
Advanced: Godot games, programming target?

This is also the sort of thing I could see having an OLED for longer battery life (+always-on info) not that I could justify the price. I don't even really need a second portable device (and my headphones situation isn't good either).

Maybe the sort of device I'm wanting is unlikely to exist, especially with some sort of battery compartment at this size and in this-day-and-age.