Maybe a 475 straight which would be about the same groove as the 470 but a straight case so the chamber wouldn't get reamed as much as for a 470.
The 470 is the logical next step up, and has been done with these rifles before but I am not really sure if it adds value to the gun. Its a Rigby cartridge too so proper for the gun but there would always be that thought in the back of my mind... they do pass proof though so maybe nothing to worry about.