Paradox only recently talked about a bunch of changes coming in the free 3.2 update and now they've announced Stellaris: Aquatics Species Pack as the latest DLC.
"Sail the intergalactic seas and uncover an all new expansion packed to the gills with new options for new and longtime players alike. The Aquatics Species Pack will rinse Stellaris with a rising tide of new content, including brand new origins, species traits, civics and a treasure trove of new cosmetics. Seafarers and landlubbers alike will agree that this is Stellaris’ most immersive species pack to date.
Just be careful, sailor…legends tell of dragons in these waters…"
When they give out more info on it we'll let you know. Just like the 3.2 "Herbert Update" there's no set date yet.