Ironically, Root's best chance might have been 1908: I believe TR wanted him as a successor but he (Root) begged off citing health considerations. Had those been overcome somehow, likely with TR's backing he would have won, albeit based heavily on carrying the industrial northeast and Midwest (he'd have been shut out in the south and possibly the great plains).
Possibly former Missouri governor Herbert Hadley might have served in a pinch. The same might be said for Charles Fairbanks. By 1916, Albert Beveridge was damaged goods. It's somewhat early in their careers for either Irvine Lenroot, Frank Lowden, or William Sproul to warrant much consideration. And Hiram Johnson was probably too far to the left for the northeastern establishment. Long story short, it pretty much comes down to Hughes as the only real option.