I don't know, I was just giving an idea, I don't even use the bookmark feature because it's only useful for audio books.
But my old player had a feature I really liked:
At any moment of a song, you hold a button down, and that moment becomes "A"
Then later you hold it again and it gives the moment "B", and the player starts playing from A to B in an infinite loop until you hold the button again or press next/previous song;
It's really cool when there is a part of a song you really like.
So making a combination for this would be cool.
Oh and another bug I noticed: In file browsing mode, shuffle and repeat are not remembered (Well, technically they are remembered but not applied until you go to the options about shuffle/repeat)
For locking, left+up and left+down both work, I think it is simple enough.
Something that would be cool too, but I don't know if it's possible to do this, adjusting the volume automatically according to the outside noise.
Like, you regularly get sound from the microphone and auto adjust the volume of the music according to the level you got from microphone, with an user-defined adjustement (-5 to +5)
So you get the volume from microphone, if it's quiet, set volume to 5(+UserAdjustement), if it's above a given level, set volume to 10+UA, another level 15+UA, 20+UA, 25+UA.
But yeah I know that's probably too much trouble, but maybe not, technically that's a possibility
Also the programmed shutdown feature, it stops automatically after X minutes, but it would be great to set "shutdown after X minutes without clicking button", so that if you fall asleep the battery is not dead when you wake up
And also be able to adjust screen brightness, if that's possible
My old player also add A3D support ( https://en.wikipedia.org/wiki/A3D ), and I think it was cool, but I don't know if that's an easy thing to add
Well, good luck