Federations DLC alongside a 2.6 patch threw a whole new life to Stellaris. The expanded Federations make the universe more divisive: from the Hegemony where Overlord gets the biggest bonuses through Galactic Union (space EU) to Research Cooperatives which ultimate goal is... well research.
On top of that You get Galactic Community, so basically the biggest game-changer since inclusion of mid-game crisis with Khan. There is a lot of words that could be said on the inclusion of Space UN in this game, but one is for sure - it completely changes the whole dynamic of game from early through mid and up to endgame, with resolutions pushing the whole map towards specific ethics and giving bonuses to all empires and breaches providing huge penalties, emergency meetings during crisis times and so on.
And that's not even mentioning Origins and new Megaconstructions! One of the best DLCs to Stellaris.