This issue a is a rabbit-hole with so many issues ...
Consider the person putting it together:
Beautiful routing. Don't keep a rats-nest like Input Club's PCBs.
All user-solderable components well laid-out. For instance, through-hole diodes in rows and columns and all facing the same direction, or even in pairs.
Avoid putting through-hole solder pads too close to SMD components where they might get damaged by clumsy handling of the soldering iron.
Don't route across a pad. If the pad gets lifted when desoldering, it should not have to be repaired by also having to bypass the pad: it should already be bypassed.
Make sure that
if you need multiple footprints for the same type of component, that they are similar. (There are PCBs out there where some key positions fit Holtite sockets and others don't)
Some artistic flair is always appreciated!
If you have the microcontroller on the PCB:
Consider
USB breakout, with USB Type C. Or a through-hole header in addition to USB port on the PCB itself for those who'd want to customise.
Fuse for overcurrent protection
Unless you are only using a robust ĩC like an AVR, consider ESD protection. (which is not the same as overcurrent protection)
The usual rules for grounding USB shield, USB C identifying resistors, routing power lines, routing, communication lines (D+, D-, IēC), making ground planes and placement of decoupling capacitors apply. You would need to learn them.
Features:
People expect a couple layout options, even if you don't offer different plates, people could modify them.
Always have holes for switches' fixing pins
Reset button for entering the bootloader.
Things like SMD LEDs, flexcuts or whatever is not a quality issue, but preference.
There is a whole lot of discussion of various design issues in
The Living PCB Design Thread.
