Here is a wildcat idea:

The 450, 458 and 50 are all interesting cartridges with decent power but they do leave some capability on the table due to the limitations on the bolt strength. I believe all are limited to the mid 30kpsi, if you move to a WSM case head and bolt like Olympic Arms has done for their WSSM uppers you would push your chamber pressure capability to 60kpsi. Not too long ago I looked at a wildcat using this upper design, it just so happens that if you cut down a 300WSM case to 1.615" you end up with a case ready to take a .510" bullet using a 300WSM sizing die. The flaring die could come from a 500 Linbaugh, the taper crimp die would need to be custom. The end result should be an AR round that exceeds the capability of the 500 S&W. I'm sure others have looked at doing this before as we'll, I've even heard Olympic Arms is working on it but have not heard an ETA for any product release.