[Update]: Stellaris' freshly launched Overlord expansion has now reworked vassalization and spiced up the mid-game.

In an effort to spice up the rather unimpressive vassalage mechanic and make diplomacy more interesting across the board, Stellaris will be launching the Overlord expansion this May 12th. As the name might imply, Overlord will focus mostly on offering you new ways of projecting your power and asserting your will over the universe, be this for the betterment of all or purely yourself.

You can expect to see an overhaul of how diplomacy works along with new forms of hostile 'negotiations' through the use of espionage operations, new origins and megastructures to help spice things up, and most importantly, a massive rework of vassalization as a whole. The goal here is to make subjugation a deeper and more engaging system with benefits for both parties, including the player should they end up on the wrong side of a galactic tyrant! 

While it doesn't show off any gameplay, the freshly posted and rather pretty cinematic trailer should give you a bit of an impression of what the Stellaris team is trying to do with Overlord. Have a gander:

As is tradition, the Overlord expansion will also launch alongside a bunch of free changes and additions in the form of Update 3.4 - Cepheus, though sadly the details are currently few and far between. However, with the expansion launching in a mere two weeks, that is very likely to change soon.

So if you would like to catch the patch notes as soon as they're released, as well as learn more about the Overlord expansion, you should head on over to Steam.