There's probably room for a new big bore between .510 and .577. Maybe base it on the 505 Gibbs case with a bore of .525 ((525 Gibbs)? A 650g bullet at 2400 fps gives you an SD of .336, Penetration Index (based on the "Any Shot You Want" formula) of 130 (better than the 416 Rigby shooting a 400g bullet at 2400 fps), kinetic energy of 8331 ft/lbs, Taylor KO factor of 117, and recoil in a 13 lb gun at a sedate 118.69 ft/lbs.

Or Maybe a 475 Gibbs shooting a 500g bullet at 2700 fps? Gives you a kinetic energy of 8091 ft/lbs with less recoil than the 525 above, flat shooting as a 30-06 with a little more recoil and knockdown power.