>>281557939
You can make completely custom factions.
The DLC adds some premade factions, but also makes all their things available to use in custom factions.
This includes not just their looks, but also all the traits cultures and stuff.
Here's some images, all the things with those extra two letters ("PF", "EA" are added by DLC).
https://imgur.com/a/WlSpodP
And most importantly, it also adds those tomes of magic, which are basically your tech tree and give you spells to cast, units to build etc.
So the DLCs give you more variety, but all actual features that came with the updates have been added to the base game, so until you run out of things you want to do, the base game is perfectly fine.