The FBI and DEA are buying a G II based .308 from Remington Defense, or TA least they were till the first production guns got recalled.
POF makes a small frame AR-10 and MEGA released one just before they were acquired by ZEV.
Once a cartridge is finalized a true “mid sized / AR-12.5” is certainly possible. The custom sized 6.8 SPC LWRC did fir a Middle East client is an example of how the design can be scaled.