I once made a wildcat cartridge years ago 32 Xtra Special ,the cartridge was based on a 32 Special ..I had a Marlin 336 in 32 Special ..had it re-chambered to 308 Win ..Seated the 32 cal. bullet deeper (like the 307 win) which gave me more capacity than the 32 Special ...used 307 Win loading Data ...really could not tell much difference than the 307 Win..just fired a 170 gr. .321 bullet as compared to a 170 gr .308 bullet...sold it at a gun show to a very happy camper,,,
Its hard to improve on the cartridges today ..the 338 Xpress seemed like a great concept ..but Remarlin is letting it die...
If you want fast in a Levergun you have the 308 Xpress , if you want flatter shooting neck it down to .260 (6.5)
If you want fat instead of flat you have the 444 Marlin , 45/70 Gov. 450 Marlin....if you want fatter go to a 50/70 or a 500 Marlin based on the 450 Marlin
a 500 S&W lever would be nice ..16 in barrel or Rossi start rollin out he 454 in the 92 Pumas
But a 260 (6.5) Xpress would be cool...140 gr. RN or eraser tipped bullets ..great SD running out of barrels at velocities equal to the 6.5 X55