That would be interesting. They should have it that you cant any use city-state delegates for world leadership, and you earn your votes based off previous resolutions passed or supported. Probably, a rank system in which diplomatic support tips you one way or another. Reaching a threshold earns 1 delegate from that civ. Something along the lines of:
Rank Points, you keep these points as long as they are in effect:
Open Borders +1
Trade Luxury +1
Trade Route +1
Fulfilled Request +1
Defensive Pact +2
Declaration of Friendship +2
Warred together +2
Bonus Rank Points, these are added to your pool:
Aid passing resolution +1
Propose favorable resolution +2
Liberate City +1 (Only applies to city once)
Recall to Life +50
Special points:
At war -5 (negative points only when at war)
Destroyed civ -50 (only if civ is recalled to life)
Captured a city -1 (Only applies once per city)
Use nuke -10 (to all civilizations)
Sabotage a resolution -1 (to that civ)
Differing Ideology -5 (until ideology changes)
Earning one delegate from a civ, requires a rank 10.
Earning a second delegate from a civ requires rank 20.
Earning a third delegate from a civ requires a rank 35.
Diplomats can negotiate 2 delegates from a civ.
First to research Globalization gains a permanent +5 rank to all civs.
At least this way, it relegates city-states to helping win your goal by becoming host or supporting other resolutions, but still focuses on your relationship with civs over the game. This also makes ideology important, but not always necessary.
Paying off people to vote you in is too realistic.