This kind of touches on an idea I had a while back about 'Monster Races,' which while a bit on the nose for a title, gets the point across.
Everyone in Stellaris is kinda playing by the same rules, and that's a good thing. It means that diplomacy is important and war is structured and deals can be struck between the winners and losers of a war that actually have some depth. But most other Paradox games have (or through DLC, end up having) some other types of factions that play by a different set of rules, who aren't held to the same restrictions, maybe have their own sets of restrictions, and whom you can interact with in different ways. Not requiring CB to annex pagan territories, for example.
I really hope (and expect) to see this applied to Stellaris in the form of 'monster races.' So that we can have the structured diplomacy-centric empires who follow their shared sets of rules and negotiation, and then we can have empires that follow a different set of rules. Mindless insect races bent on expansion and consumption, unintelligible machines bent on replication with no empathy for the living, grey-goo infestations that take over huge chunks of the galaxy. Things that you can't reason with, things that never stop being a threat, things which could appear and erupt at a moment's notice, engulfing a remote part of the galaxy and growing into a force to be reckoned with. Things that in many ways act like an empire, but can't be bribed or appeased, or held to peace terms. Tropes, certainly, but good ones, reasonable ones, that I think belong in the game. Space should be a little scary. Not everything capable of hurting you should be something you can reason with.
BUT ANYWAY, more to the original topic's point; a lot of games start in a 'cold war' state, where you can freely attack anyone in neutral territory without declaring war on them, and of course, you can destroy any foreign ship in your territory. I think this makes some sense, but it does limit you a bit. What if you stumble into alien space without realizing it? What if you attack one of their ships there? Should that immediately establish diplomatic relations and put you at war? I don't think so, at least.
I think that establishing diplomatic relations with other alien species should be a process, and not something we take for granted. And I think that agreements about territory and the like should require relations. How do I know what space that lizard considers hers until I've spoken with them? And how have I spoken with them if I don't know their language?
It could be a little frustrating at first, with ships stumbling into eachother's space and tensions rising, but that could just be an incentive to establish communications. Or if you're xenophobic, incentive to attack their planets without ever breathing a word to them.
