War crimes could be a great addition for Diplomatic victories
Ok so obviously committing war crimes goes completely against the ideas of diplomacy and I doubt we'll ever see Firaxis or any other company intentionally let you do them in any civ-like game, but recently I realized that prosecuting war crimes could help diplomatic victories feel more thematic.
Across all games, diplomatic victories have always had some kind of connection to the UN or some "World Congress", and the main objective of many of these organizations in real life is to ensure human rights are respected and stopping major violations of them. It would be interesting if one player is warmongering that you could start a tribunal with the other players to put major sanctions on them to stop the war. This could be a better way to stop military superpowers through means other than war. It would also actually be more in line with the idea of "diplomacy" than just being good at elections, the more interesting part of diplomatic victories in civ 6 (at least for me) have always been contributing to some world project because it actually feels like you're building a diplomatic community instead of just seeing what other players voted for.
This could be made more interesting with the new system for Influence and diplomacy in civ 7, where to start these motions and push them forward you would need Influence. This means you would actually need a resource to do it and potentially if you're the one with the big military you could pass over such restrictions if you're influential enough (sadly not much unlike real life).
The main problem is that, well, to prosecute war crimes committed by other players you need to make players able to commit war crimes. As you've probably noticed from my language before this could be simplified to just "doing a lot of war" but still it is hard to distance it from real war cases were actual war crimes and similar offenses were prosecuted. I do hope you at least found the idea interesting.
TLDR: prosecuting other player's actions in war would be an interesting mechanic and a better thematic for a diplomatic victory, though there could problems with irl parallels