If I was in the market for a combination gun I'd be looking for something European in flavour, since that's where they originate. I think something in 16ga over a 6.5x54/6.5x55/7x57R would be just about perfect to me. I don't know why you would particularly want a 12 over a 16 for the jobs we're looking to do with the rifle and a 308 or 30-06, as much as I like them on their own, just doesn't feel right in such a gun. Of course, a drilling in 16/16/7x57R or 16/16/9.2x74 wouldn't offend me either.
Bastards...now I have to go surf the TradeEx site.