I know you said no 458WM but I would consider it. It has the simplicity of a standard caliber rather than a wildcat. And it seems to be flexible with how you load it. I have been loading mine with 325 FTX bullets from Hornady over RL7 and honestly I have shot 45-70's that kicked more. You can get the 458WM kit by ER Shaw at midway or brownells for a couple hundred bucks and get gauges and an extra wrench.

You could also do a 458x1.5 I think it is called. It's just a shortened 458WM.

I have also been kicking around ideas of a 44 caliber wildcat. Was basically thinking of a 44 Auto-Mag "Magnum". Since the 44-AM is basically a shortened 308 thats necked up to 44 and then had the neck inside reamed to accept a .430 bullet. I was thinking of doing the same but not cutting off the 308 but leaving it full length. Basically making a 444 Marlin Rimless. It would be a lot of work to form brass I bet, but not too bad. I would be willing to bet that if it were loaded with a Spitzer type bullet like a FTX or TTSX it would feed from a SA Centerfeed Magazine without too much modification.

I don't know if any of that helps but just some thoughts. :)