Depends on the game, depends on the players. Many games on Steam have a very wide range of mods ranging from entirely non invasive music in the background to character voice packs to entire gameplay total conversions.

For "normal" Stellaris there are things like Leviathans Events Xtended, More Events Mod, Zenith of the Fallen Empire, New Ship Components, Ancient Cache of Technologies, and many more that offer different kinds of challenges be they edits to existing ones, novel additions, or simply more ways to build ships and have things to fight. Then there are total conversion mods like Star Trek New Civilisations that lets you play Stellaris but as Star Trek. There are several Trek mods like this. There are also Star Wars total conversion mods, as well as ones for Mass Effect and a few other franchises.

X-Com 2 is similar to this. The Hive adds in new chrysalid mobs to deal with while Long War 2 flat out changes how the game works with its total conversion and novel class designs.