Almost anyone would have been more conservative than TR on domestic issues. But Root was a brilliant man, so some sort of accommodation would have been made. The much bigger problem was Root's close association with Wall Street: that would have made him a tough sell in the Midwest and Great Plains. TR would have had to campaign heavily for Root. Then too, when approached IOTL about the 1908 nomination, Root begged off citing health/medical concerns (don't recall specifics). So...long story short, I'm not sure Root would have been the choice. Perhaps a meeting of the minds could have reconciled TR and Charles Evans Hughes, or perhaps Missouri governor Herbert Hadley might well have been the choice instead.