might be hard to do for small size, but a cool option would be to have BT audio (the good protocol, apt-x, at least) be present in some way. I can see 2 links, possible: a link from the player (phone, etc), and a link from the amp to the headphones.
now, from the phone to the amp, it could be useful if you want to avoid wires between a phone (maybe in one pocket) and the amp module that might be elsewhere on you.
the other link method is when you are using a dedicated mp3 player but it does not have BT and you want to use BT phones. in that case, you'd have this box take in line-based audio from a cord and transmit audio via BT to the wireless phones.
I doubt that both links being BT would be at all useful
but being able to enable the BT link on either end would be pretty 'trick' and I've not seen any box offer that, before.
when you are transmitting BT to phones, your amp is mostly bypassed and you are just a transmitter, now. but if you look at the other case, where a BT sender sends to your device and you have nice expensive wired phones on the amp module, that could be a reasonable use-case.
also, for the psu, not sure if you are going to allow usb (micro?) charging as well as wireless charging, but if you allow wired charging over usb, you could also (maybe) have a usb dac in the box and add yet another function (fiio has a few small boxes that act like usb dacs but are mostly phones amps.)
maybe too much stuff to fit in a small box (lol) but just brainstorming some neat ideas, in case any sound interesting to you.