Suggestions

[ApocalipsA]

1. The Center of the Galaxy?
In real life is the center of the Galaxy dense with stars and at least one very massive black hole... that is essential for life in the Galaxy... at least i think so...
For example the milky way has a big black hole in the center called "Sagittarius A". And there are much more stars . In stellaris the center is empty...

2. Primitives
Why you are not able to prevent this primive civillisations for ever to go to the stars? - So you can study them for thausend of years +10 society research points yeaah!

3. More Battle Tactics
The big ships with tons of hull points and shields shout have a new tactic option like:
Breaking trough the space like a black hole and appear in front of smaller ships and make them to crash in them. So the corvetts will explode and the battleships will just take some dmg to the shields.
But appearing not face to face, i mean they shoud crash in the side of the battleship... my english so bad i can`t describe it better

Titans may have new active tactic like retreat. They shout be able to shock the enemy and make all shields deaktivated for some seconds. Like an EMP today on our Citys... no light, no electricity, back to medevial age.

4.More technologies
Let´s push gas gigants near to the star to be able to mine a lot of minerals from the hot core after all this gas burns out.

Type 1 Civ. Mastering all ressources of your Planet
Type 2 Civ. Mastering the Star
Type 3 Civ. The Galaxy is ours.

Sorry for my english

 

[Spaceception]

Having Type 1, 2, and 3 milestones would be very cool, and fit within Stellaris' sci-fi tropes and such. Maybe there could be new (rare) technologies to get once you reach those points.

Adding a center to the galaxy has been suggested a lot afaik. Not sure what current plans are.
But what about the opposite of that? More galactic clusters outside the main galaxy. Not hidden like the L-Cluster, just harder to get to.

Making your own Chthonian planets could be cool. Adding to that: Maybe there could be a new mid-game megastructure that allows you to strip-mine planets with that modifier (Maybe modifiers like Carbon world as well). You can only build a couple at a time, and you can move them to other planets after a while if you need to. Could help with the problem of planets not having enough mineral districts, and is a reasonable thing to expect in a galactic civilization. As well as a halfway point towards a matter decompressor.
A similar thing could be added with solar panel networks around highly populated planets (>50-75 pops), a Dyson cloud around inhabited planets with solar arrays, and some extra districts (as the halfway point for Dyson spheres).