On the casus belli thing and other interactive diplomatic options. It would be interesting for something like this to develop over time in the game that demonstrates how politics between empires evolves in to something more intricate and advanced.
Like in Civilization games, you reach the United Nations near the end game to show how global politics has gone beyond simple war rararara.
Would you like to suggest this more intricate and advanced diplomatic system? All i can see are empty suggestions without going into any meat. To compare with Civilisation, Distant worlds pretty much have a better diplomatic system than Civilisation games, with their active trade system and blockades. As for the United Nations, in Civilisation, all it is, is a bland nonsensical victory condition.
One thing that could be fun is an additional layer of exploration beyond exploring the galaxy. What I have in mind is something similar to Warlock: Master of the Arcane. In that game you had the main map or world but, there were these portals that you could enter into another world or dimension filled with alternate landscapes, creatures, and challenges. Something like that would be cool in DW. So you could have these Wormholes that led to other galaxies or dimensions that contained different monster classes and types as well as different races and types. They could be further characterized by extreme space hazards like the bad lands in Star Trek Deep Space Nine. It could fit into the story as well....for example...suppose a race stumbled onto a wormhole that led to the place where the Shakturi returned from. These other galaxies or dimensions could also hold special techs found at planets, asteroids, or on abandoned ships that can not be researched.
This type of exploration could be a late game technology function unlocked by a tech.
Rather ironic in that in playing a space game, you have a sudden desire to be whisked away into an alternate universe full of strange landscapes, creature with monsters and different races and extreme space hazards. Why, that sounds like Distant Worlds! Also I dread to think how the AI will be scripted to react to those circumstances.
Perhaps WAR should require certain triggers, whereas skirmishes and unruly behaviour 'tit for tat' are par for the course and not deemed war. More in line with how the world works... unless it's North Korea who are in a constant state of war without being at war.
They already are, hence why you can invade worlds without declaring war, and why the Gizurean are always shooting at my ships patrolling my mining stations within their territory, and then they have a reduced relationship from "our past dealings with you have been terrible". And in real life even minor skirmishes have huge chances of escalation into act of wars; in fact they are a symptom of being at an uneasy ceasefire, it is certainly not how the world works.