Lets see, what are some key features that would make this DAP a winner...
UI
Simple intuitive with gapless playback. Also tack on stuff like shuffle, playlist support and genre artist and album tag support for easy browsing. General file type support such as FLAC ALAC WAV and mp3. Most importantly NO UI BUGS PLEASE
Battery life
15 hours minimum 24 hrs or more would be great though
Storage
SD card compliant minimum 64 GB's, 128 preferred though
Sound
I'll put it as bluntly as possible.....It should sound as close to absolutely nothing as possible. In other words it would just disappear and let the gear it's driving stand out. If the gear plugged into it is warm then it sounds warm if it sounds analytical then it sounds analytical if it has a V or U shape then it will sound exactly like it was tuned to sound like. There should also be as little roll off in the upper registers as possible. A lot of mainstream DAP's play it safe and roll off the highs and I personally hate that. Oh and since I'm asking for extension up top give this lady a well rounded bottom end too. If source demands a deep earthy rumble I want to hear that rumble in as much detail as possible.
There's plenty other stuff I could mention but those are the big ones for me. Anything else would just be added gravy on the steak. Sounds simple enough in my book but for whatever reason none of the boutique DAP manufacturers are yet to deliver anything worth mentioning as a complete package.
Whoever is with me gimme a thumbs up