I'd love to see an Allegheny, but a more likely "next big steam" would be a 2-10-4. Used by a fair number of roads, and the C&O T-1 was essentially copied by the Pennsy for its J1; yes, the Santa Fe had larger 74" drivers, but one could imagine that once the basic 2-10-4 chassis was built, having a 69" and 74" driver version wouldn't be too difficult (the scale difference would be about 1/32").
Once you've done the Berkshire, the C&O T-1 2-10-4 is just a longer version (almost literally - it was developed as a "stretched" version of the Erie Berkshire, albeit with 69" drivers instead of the Erie's 70").
John C.