AK = cheap, mass-produced-capable, simple to operate and effective.
How about a KP-15 receiver with a simplified upper? 1-pieced stamped hanguard, no dust cover, 11.5 in HBAR and a simplified iron sight assembly? As for optics just toss a short pic rail on it.
Such a rifle would be:
- cheap. Polymer-casted receiver ( apparently they could be churned out real quick, like it takes a couple minute to mould the entire thing into ond full assembly); COTS components etc. A heavy barrel would take little time to machine, and also improve balancing/ recoil control for the user.
- Manufacturability. Again, polymer casting, and relatively cheap stamping/machining of parts.
- Ease of use. The AR15 is an extremely popular platform.
- Reliability: well it's the AR15. Lube it proper and done.
As for historical examples the AR18 would be my take. It was designed to be built from a couple of lathe and tools that even insurgents could get their hand on. IMO it's the closest Western analogue to the AK47 bar purpose-made insurgency/wartime guns like TRW LMR.