These Stellaris mods will change your gameplay dramatically. Know what you're getting into before adding these!

NSC2: adds new ship classes, weapons, and ship AI. See https://nscmod.com/ for details. If you install this, also install the two addon mods for races if you installed those. Many ship sets (like Kurogane) include NSC2 support out of the box. Also grab the Mass Effect shipset if you get this and want more ships from an entirely unrelated game!

Extra Ship Components: adds a bunch of new weapon types, addons and gewgaws for your ships. Good if you think refitting ships in Eve was overly simplistic. Works well with NSC2. Be sure to get applicable addons for this as well.

Expanded Traditions/Unique Perks: think the basic tradition trees are too, well, basic? Well, have a bunch more. Balance? Who needs it, we got galaxies to burn. The Unique Perk mod in particular can become wildly overpowered and will slow your game to a crawl with multiple infinitely expanding empires in the late game, so caveat emptor.