Yes, that's the problem of diplo victory - I never seen any good suggestion for it. Forcing other civs to vote for you is lame and any city-state based mechanic is not connected to diplomacy itself.
I believe the key problem here is - the other players (especially human ones) are opponents, not just another game mechanics. Being able to win through diplomacy conflicts with this, it's like "oh, you speak so well, you won".
Diplo victory never found a home in any of the Civ games
But there's nothing wrong with city-states casting votes as they are in the game. Actually, as they are now going to play larger roles in the game, and the mechanics around gaining influence over them much more refined, it actually makes even more sense to count their votes now.
That said, I still think Diplo is the one cut and city-state based mechanics will be put in its place in a VC that's sort of like diplo, but different. (I call it hegemonic victory) basically VC by having the most worldwide influence. This would include Cs votes, influence or power over other Civs, etc. This is a VC more well suited for modern hyper power superstates that was never really measured or acknowledged in prior Civ games.
On that note, since they have confirmed they are only adding 1 new VC,
I also feel religious victory is anachronistic as the new VC to add. Modern society has largely moved past religion. It seems more like a VC more well suited for a Civ game made 200-300 years ago. It would also go against Ed's mantra of introducing new things in each age. Assuming we follow G&K logic, religion will fade around the renaissance.