I often considered this.
G&G SSG-1 is an attempt to get there https://www.foxairsoft.com/airsoft-guns/electric-rifles-aeg/m4/g-g-ssg-1-usr-speedsoft-airsoft-gun/
I'd say you need a v3 gearbox (common but much more robust than v2).
Translucent mid cap mags, compatible with existing mags, probably mp5 as they (an ak mag pouch can hold 1 * ak mag, 2 * m4 mags or 3 * mp5 mags).
I'd also go bullpup so that you can get a full barrel into a subcompact aeg.
might even channel the BBs from in front of the grip like on the conventional platforms to the rear.
Might even have an internal magazine big enough to last a match, maybe that can be reloaded or extended with external mags. Maybe several internal tubes in either parallel or in a revolving setup.
Safety trigger, possibly also an external safety, but one that can be locked in unsafe (way too many times someone has handed back an airsoft to me in safe so I've been killed in the field because the gun wouldnt shoot)
Selector switch possibly ak layout, bumped the m4 one by accident way too many times.
If it used external magazines, side feed would be awesome, meaning you could lay down without having to switch to short mags.
Ok, so just brainstorming: what if it had parallel gearbox n barrels, meaning you could, with the selector deciding which one, meaning you can have DMR fps when on semi?
I guess I should say quad rails, but I also feel like that goes without saying.
I'll probably add to this later^^