I think he means things to add to the todo list.
And I'll reply to it: players can refuse to participate in a Congress and can refuse to cede a city whether they participated in Congress or not. It has been this way for a few versions.
As to improving Congress, the entire mod is a work-in-progress and every single feature is being slowly improved as time and feedback permit. Please be patient and try to assist development of the mod with positive, constructive feedback - specific ideas for implementation, with detailed justification where needed, are always welcome.