Leyrann
Deity
Could natural disasters be linked to a World Congress mechanic based around humanitarian relief?
Let's stop with the world congress. It's ridiculously arbitrary and at the same time too powerful and absolutely useless.
That doesn't mean there's no need for late-game advanced diplomacy though, we just need to build it differently. World congress can be used as a base if we know the fundamental problems it has. Which are: First, it makes no sense to have all nations in the World Congress; the closest thing we have to that is the United Nations, and even the UN has some nations not partaking, while it also suffers from a sense of powerlessness at times, for various reasons. Second, the World Congress required you to do resolutions, which could result in the most useless stuff happening. At the same time, it's very bland in it's victory; gather enough votes through population or city states (IV and V respectively) and vote yourself world leader.
The first thing to change, then, is the membership of the World Congress. There are a lot of real-life examples you can look at; NATO, ASEAN, EU, Warshaw (sp?) Pact, Arabic (Arabian?) Liga, etc. These are all diplomatic pacts that do various things together, like trade contracts, wars or embargo's on non-members. So, instead of a World Congress, let's implement Diplomatic Leagues, which can be founded by major civilizations, which other civilizations (major and minor) can join. Those Diplomatic Leagues do various things together; how much depends on what the League members agree on.
And the second thing, is to improve the power it grants, the frustration it may cause, and reduce it's occasional uselessness. Those can all be swept in a single change: Voting no longer happens at set intervals, but rather when a major civilization calls for it. They can vote on things like forming a defensive pact, whether the League declares war as a single entity, etc. Of course, there are limits on voting; if something is voted away it can't be voted for again for X turns.
And lastly, the victory. To make Diplomatic Victory deeper, we should look at what it represents. And that's pretty simple; you're so diplomatically powerful that you are the one that decides what goes on in the world. So how do we achieve this? By having the victory trigger if your League is large enough (e.g. 80% of the world's population) and you have been the leader for a long enough period of time, with that period being significantly longer than the minimum time between leader change votes, so that you need to win several in a row.
See also the link in my signature where I worked this out in more detail.