JeepLIFE,
you'll likley have to stay within the same pressure limits, cartridge dimensions, and rim diameter. The 38/55 is basically a "blownout" 30/30 (although the 38/55 came first and was used as a basis for the 30/30) so it would be the easiest to convert to.
There is also a few wildcat cartrides based on the 30/30, one that comes to mind is the 35/30 kinda like a "ackley improved" taper with a necked up caliber, although it was meant to be a modernish improvement in the lever action rifles of the day, I think that it's similar to the 38/55 in in actual preformance.
Just FYI, I hunted elk with a friend a for few years that used a Remington model 8 in .30 remington, (basically a rimless 30/30) and he had success with it, typically he hunted the thick timber, while the rest of us hunted the more open country, but he never lost one that he shot at. usually after opening day the Elk prefered the thickest stuff they could find anyways, where a flatter shooting gun really didn't make much difference.
Rvtrav