If you want to go big with the AR platform, why not 450 Bushmaster or 50 Beowulf? You mention short barrel/pistol so I presume you don't want a big bore carbine. So scratch that option.
If it is about having a better pistol in 45, why not choose a double stack 1911 (a.k.a. 2011) with a can? Or a full size polymer/striker in 45? You are going to need to use pistol mags anyway, so why pack the extra bulk and weight of an AR if your preference centers on the 45? I guess my question is what the AR platform brings to the table if your goal is a 45 ACP pistol.
I think most people agree that compared to the 9mm, 45 ACP is at it's best (ballistically) when you are up close and personal. Also inherently subsonic thus easy to suppress and it's quiet. Under what circumstances is an AR pistol in 45 practical? It isnt. Also costs more to build, costs more to feed it.