First, I should NEVER be penalized for breaking a treaty when it isn't my fault. If I trade GPT to a civ for a technology, and that civ is killed off by another civ before the 20 turns ends, that's not my fault.
If I'm trading GPT to a civ, and that civ then attacks me, it's not my fault.
If I'm trading GPT to a civ, and that civ loses it's harbor city to another civ, it's not my fault.
The idea behind those things is great: if I break a gpt deal, other civs are less likely to give me gpt deals. In practice, it sucks. The last 3 games I've played, gpt deals have been nearly impossible, and it was never my fault.
Also, I'd like a better advisor. If I am about to start a war, instead of just a warning "Are you sure" screen, tell me which treaties I'm about to break. Warn me that they have a MPP with my trading partner. Along the same lines, warn me on the turn before trade agreements, peace treaties, and other agreements expire.
Also, I'd like to see the UN used differently. The owner of the UN should get to call votes to impose sanctions. If a country launches an aggressive war, you could have a UN vote to impose an embargo. If the aggression continues, you could have a UN vote to declare hostilities against the aggressor. The embargos and military alliances formed at the UN would be free of cost to the owner. Also, you should be able to impose sanctions for using nukes (or even building nukes maybe). Also, maybe allowing sanctions for excess pollution.