The levels of DLC possibilities are...You guessed it.

[Wizzington]

The AI will only threaten/insult you if they think they can take you. Or you insult them first.

 

[Wizzington]

That sounds promising. I hope you will also be able to make friends in later game when you are clearly the dominant species in the galaxy.
Like promising to leave someone alone (guarantee their independence) and they are actually in friendly terms with you.

In many other games the AI turns hostile by default if you own 50% of the galaxy. Yet nobody really forms an alliance against you... they just don't like you for whatever reason ... even if you were on peacful terms since turn 1.

The AI won't turn on you due to size alone, but rather will react to conquest by forming alliances and federations to counter the expanding empire's power.

 

[Wizzington]

In those other games, they program the AI to "win the game" rather than just survive. That means when you are close to domination victory in 4x games, the AI is told to turn on you. It doesn't necessarily make sense except in game terms. So it's not a simulation. Ck2 type games tend to favor simulated responses more, where the reactions of the factions in game are more natural or logical. Or at least, can be countered by things other than eliminating them.

Games like Thea show off the potential of changing the gameplay with Eastern European card based tactics, and removing the "domination victory goal" entirely. I find that refreshing, even though it can't really be called a 4x any more. The Last Federation from the programmers of AI Wars, also changed the gameplay to be something more interesting.

I really like The Last Federation. It has its flaws and it certainly isn't a 4x, but it has some really interesting mechanics in the way the game reacts to the player trying to win. I feel strongly that the AI should avoid 'gamey' diplomatic reasoning as much as possible, everyone turning on you because you're winning just shatters your immersion.

 

[Wizzington]

In Stellaris there's actually a 'talk back' weight, so if you insult an AI they're more likely to insult you back.

 

[Wizzington]

A dlc that allows tall empires. A tall empire would build more space stations, colonize more asteroids, colonize fewer planets.

This is already an option to some degree, since empires with larger populations have slower tech gain and you can build outposts to extend your borders instead of colonizing.

 

[Wizzington]

The goal is more 2) than 1), and yes it's moddable.