I was thinking back to previous iterations of the series and you would always have a way to change suzerain of a city state, but in Civ 7, there is still no way to accomplish this.
I think the most straight forward way to do this is by using Influence on a Sanction specifically for removing the suzerain of a city state.
The details of how this would implemented would be up to the devs, but I have some basic thoughts on how I think it could be implemented:
- If the sanction is successful, then the city state becomes available again and the previous suzerain is not notified who completed the sanction
- If the sanction fails, then the city state remains unchanged, the Suzerain is notified and given some influence, relationship decreases