New gameplay and assets can be added through Mods from the Steam Workshop, beware though as Stellaris updates regularly and not all mods are kept up-to-date.
Stellaris has a basic mod manager built into its launcher, once mods are downloaded its important to ensure your mod order is correct - otherwise you may find your game will present errors.
Plentiful Rich Galaxy
This collection adds:
more portraits and species backgrounds,
traits (including machine traits),
more planets & origins,
expanded traditions,
new megastructures & origins,
UI Overhaul.
https://steamcommunity.com/sharedfiles/filedetails/?id=2417695428
Gigastructural Engineering & More (2.8)
Planetary Diversity - Gaia Worlds
Planetary Diversity - Exotic Worlds
Planetary Diversity - More Arcologies
Planetary Diversity - Planet View
Planetary Diversity - Planetary Habitats
Planetary Diversity - Space Habitats
Planetary Diversity - Reworked Habitability
Planetary Diversity - Shroud Worlds
Planetary Diversity - Unique Worlds