William's mention of the .303 British brought to mind an interesting and fun approach to a very useful levergun wildcat.
Key here is that both .303 rifles and ammo are available surplus at very low prices. You could buy a .303 surplus or from Gibbs and 500 or 1000 rounds of ammo very cheaply. You would have a very tough knock around gun in the .303, and good fun and practice in converting the ammo to cases.
The .303 rim is right between the .444 and .45/70 in diameter, and the case length is short enough thus it sould fit many of the Marlins and Wnchesters. With the body taper blown out and a steep shoulder a .375/.303 should be able to improve on .375 Winchester numbers for instance, but you could basically pick a bore diameter. RCBS lists a 22/303 Improved, 25/303, 6.5/303, if you want to go smallbore.
Could be FUN!