Things I wrote and submitted in the form:
I can't peace out like in EU4. This happened twice to me in recent games. In the first, a fallen empire awoke and declared a total war on me. They were impossible to beat (total fleet power in the MILLIONS when I was barely 100k total), and yet I couldn't surrender and become their vasal. The game basically decided I'd lost, which was totally frustrating. (See
https://forum.paradoxplaza.com/forum/threads/awakened-empire-kills-my-game.1718842/#post-30042162)
The second instance was more annoying than game-breaking, but it still should be dealt with. I had a defensive pact with an empire on the other side of a mutual rival. I couldn't get to their space directly. My ally had a rebellion happen to them. Since my fleets couldn't reach their space, there was no way for me to participate in the war (and I didn't really want to anyway). Yet because I was technically "at war," I couldn't diplomatically subjugate a couple of nearby, weaker neighbors. Also, because I was a "participant" in the war with a massive, unscathed fleet, both the rebels and my ally were taking my fleet into consideration for relative fleet strengths, which made the war last A LOT longer than it needed to! I would've rather just peaced out in the first month (or paid some sort of penalty/fee to not go to war at all, like you can in EU4) than have had to sit and wait for them to resolve their rebellion before my diplomacy became unrestrained.
It's really annoying (and immersion-breaking) when a fleet that was just a few days away from a hyperjump has to respool their FTL engines the second they enter combat. If they were only 4 days away before combat, WHY are they 30 days away once combat starts?! All ships should begin recharging their FTL dives the moment they enter a system; once they're charged, they should be able to make an emergency jump. Yes, that means little, annoying fleets will have greater survivability, but it would also make FTL inhibitors actually DO something--they'd prevent enemy FTLs from recharging while inside their aura.
It would also be nice to have a "run away" order for fleets so that they would keep moving toward the place they've been ordered to go even if they're being shot at, instead of turning and trying to fight. Leia's blockade runner didn't turn to fight Vader's star destroyer in Star Wars; it kept running away even while it was shooting back. Ships in Stellaris should be able to do the same. Ideally, this should be an order we could give to a fleet while it was in combat, so that it could potentially draw the enemy forces to a specific location (perhaps where I have a trap laid for them).
Waiting for my troop transport ships to catch up is a hassle, but I also really like the existence of ground combat, and wish it would be expanded. Perhaps there's a way to split the difference. like with space marines taking up ship slots.
I think ships need a contingent of space marines to actually board/take over enemy starbases (and ships too) rather than destroying them. It should be a starting tech, but something that takes up an Accessory slot so that not all ships would have them. (With later techs, they could take part in ground combat as well.)
Status Quo is better than a binary victory or defeat, but only just. A status quo in real life happens when neither side can get further battlefield advantage, but that's just never the case in Stellaris. Right now I basically use Status Quo peace when I've achieved all my personal goals for the war (or close enough and I want the war to end), but I've not necessarily achieved (or even want to achieve) the "official" wargoal for the war. In this way, Status Quo resolutions allow me as a player to get around non-conquest causus bellis to conquer to a certain extent. And while that's useful, it's not very immersive.
Status Quos tend to leave a lot of weird border gore with different systems being cut off from the rest of the empire. I will sometimes choose to invade a world or NOT capture certain systems just so that the Status Quo borders are cleaner. Doing this doesn't really make sense from a warfare perspective, so it's a bit immersion-breaking.
Sometimes my subjects have rebellions, but I as the overlord end up owning the reconquered systems at the end of the war. Why does that even happen? Shouldn't those systems go back to the vassal? When it does happen, it can certainly change my borders and my empire's demographics.
There's no way to end the war early. You have to achieve all your objectives. When you do, victory is automatic; if you don't, the war goes on forever--even when the enemy has no fleets left, no starbases to rebuild them, and no hope of winning--they will still hold out until the very last planet has troops on the ground. That's annoying, and it takes way too long.
I would really like to have the EU4 system of selecting different provinces/tribute adapted to Stellaris, where I'd be selecting exactly the systems I want (and perhaps resource tributes too), as well as any relevant diplomatic agreements. Each would cost a certain amount of warscore, up to 100%. That way you wouldn't necessarily have to conquer a world in order to take it--but the cost could be much higher if you hadn't. Basically, if war is the last argument of kings, the resolution of war should look like the diplomacy table, but with heavy weights in favor of the victor getting what they want.
I would really love to have counteroffers be a thing as well--e.g., 'instead of ceding these systems, what if we became your vassal instead?'
One-button "achieve all my wargoals" and "surrender" options should remain in place.
I feel like attrition should matter in a way besides driving up warscore. Maybe it should decrease happiness and/or productivity so that empires have a real motivation to end wars. Or maybe it should reduce fleets' combat powers. When was the last time they got replacement crew after a battle anyway? Or fresh supplies/spare parts? And if starbases experience a lesser effect, it could make playing defensive a whole lot more viable. Same question for armies, while we're at it: How do they "heal" just by going back into orbit? Do they have clone bays on the troop transports? Attrition matters in EU4 because it depletes your manpower and therefore limits your ability to sustain the war. Something equivalent needs to happen in Stellaris, even if it's not using "manpower" per se.