You can get a Lothar Walther Garand barrel in 9.3x62, but its pricey. Criterion also makes them in 6.5 Swede and .270 Win. They also produce an 18 inch 7.62 barrel. I've also heard of custom barrels in .25-06, 35 Whelan and .243 Win, and no doubt there are others. Anything within the .30-06 case length and case head diameter would probably be do-able. Gas supply for various cartridges and propellants could be regulated with one of the adjustable gas plugs.
The standard .30-06 and 7.62/.308 always seem good to me, but there's always room for experimentation. I like the Italian Tipo 2 7.62 variation. All they need is the .5 inch shorter barrel, stock and op rod. All other parts are standard, altho you need to shorten both the op rod spring and the rear handguard.