So what are you wanting to do with it?
That is the million dollar question! I want to do something out of the norm with it but I don't want to make something that is hard to chase down brass for. From your suggestions the .444 Marlin based JDJ cartridges sounds like fun. Gary Reeder's GNR cartridges based off the .405 Win case are some others I'm seriously considering, especially the .310 GNR which is supposed to fall somewhere between the .300 H&H and Win Mag.
Anyway, I haven't decided on the cartridge yet but I want something that near modern performance.