It's true that it's released pretty basic, but the core gameplay upgrades tend to come for free with the updates. Like, what Stellaris DLC content do you think should be part of the core game? Or even CK2, which has been out and accruing DLC for ages?
Idk about stellaris, but for CK2 the ability to play different types of rulers, even if their rules are the same or slightly reflavored versions of the base rulers if you don't have their dlc, is an obvious one.
What? CK2 is a game called Crusader Kings, in which you can play every Christian king (or Duke or Count or even Emperor) in the base game. Playing as Muslims, Pagan chieftains, republican patricians, Mongols, or Indians are not part of the base game by design. If you look at the original Crusader Kings, none of those features were part of it even with the expansion, because it wasn't about those things and wasn't supposed to be.
I only played this game on release, is it worth going back to in its current state?
Nah, if you're on the fence I'd wait a big longer. The biggest problems were patched in the early round of patches, and the latest update added better government and faction politics, but the game could still use a fair bit more. Probably two or three major patches before it's an exemplary game. So like a year. If you get a wild hare up your ass and want to play before then, go for it, but there's no sense in going out of your way to get hyped up otherwise.
I only played this game on release, is it worth going back to in its current state?
Mods help it a lot. Mostly the same, but much more finely tuned than vanilla with some added flavor.
The modding community is another thing that's not really very well developed yet.
I'd say things like building megastructures should have been in the base game. Someone added them into a mod pretty much immediately, and it was always something that seemed like it should be there and wasn't. Without those, there's not much possibility of building vertically.
To be honest, the megastructures suck anyway. If the "one at a time" thing was removed and they were fleshed out a bit, it could be nice. But they're pretty boring and worthless. And there's plenty of science fiction without them, so although they could add a lot conceptually if they were better implemented, I wouldn't call them necessary either.
This isn't just fluff - it's meaningful gameplay additions.
That's not really germane. A meaningful gameplay addition isn't the same as something that's fundamental to the core gameplay, and is thus necessary for the game to be finished.