Maybe not in terms of significance but I believe Canelo/Cotto, Cotto/Canelo/Golovkin, Khan/Brook, Fury/Klitschko, Froch/Golovkin would be 'bigger' in terms of popularity.
I think this a great post. I doubt Ward/Kovalev would be the most popular fight, but it would be the one with the biggest bragging rights.
I think Ward has nothing to prove but Kovalev has been destroying everyone around him on a consistent basis, if Ward can get another solid win after Smith, it is the biggest collision course we've seen for a long time.
This fight would be second to a Ward/GGG fight for me, but I would also take a GGG/Froch fight over that.
It would if either of them had any remote accomplishments whatsoever in the other ones' division. Ward is unranked at 175 without a relevant fight there. Kovalev is unranked at 168 without any fights there... Most fans would cite "dragged him up," "boiled him down," "beyond natural weight," "below natural weight," "picked on smaller fighter," etc. so I'd honestly rather see Ward have success at 175 before a fight with someone as accomplished and exciting like Kovalev would lead to "the most bragging rights" as OP says. Exact same for Golovkin as far as a Ward/Golovkin superfight goes. But that is just me and holding the majority of opinions to a consistent standard of thought, although I know that isn't the case unfortunately.
You really are a miserable one. It'd be a great fight. Just how dominant Ward has been in the past at 167 makes him stepping up to 175 interesting.
Ward schools him. Might even force a stoppage from too much punishment. Kovalev's defence is too open for Ward.