A selection of claimable justifications would really flesh out the belli system,and allow more creative play.They could really cover alot of ground here,even going as far as having justifications that intentionally increase the relations effects.
Although I really like this idea, I can think of two instances that nations used pre emptive strikes that were condemmed by the world but were considered justified and were supported by those countries people, I just cant see a reason to use a pre-emptive strike in this game. Theres nothing to Pre-Emp. The AI doesnt mass its armies on its enemies borders before attacking. The only indication you have that its preparing for war is that it raises its Defcon Level and your gut feeling since you know which regions it usually attacks. The game would need something like WMD sites or something similiar for it to be useful.
It would also need a bigger penalty (not just hits to relations), something like locking out a random commodity from the world market for a certain amount of time based on your UN standing. The lower your UN standing, the longer you cant buy or sell that commodity on the world market. The more times you commit an aggressive action while you have "sanctions" imposed on you the more commodities you have locked out. Get yourself kicked out of the UN and your locked out of the world market completely.