I like ELEX as every PB game, but like every smaller game it is possible to play it only in some limited way. If it would be possible to mod the game, it could be much more detailed.
I don't think it will ever happen, but this list is something I would pay for instead of some extras - nothing against it, but I wish DLC could be addition to the game, which makes it more detailed and more customized to different gameplays.

I don't like playing pure humans with no alterations in fantasy/or close to fantasy games, so wish:
NPCs would be divided also into factions (not just armors, but also faces and bodies) and thus could look very different.

1. Outlaws faction would have also use of elex - they would add it as an chemical component into their elixirs, but taking that would cause partial mutations to happens, so with every rank you would have face/body like half mutant. So reavers would be more mutated humans. Also those questgivers would have different sizes of bodies - some would be very small and funny with crafty hats and tools on them, or really big and scary, but maybe also stupid.
And by siding with outlaws in Abessa you would get invitation to Fort - it would lead you to outlaws - I didn't find any way how I would know where Outlaws are except by meeting my companion - this would be logical RP way why I would want to get there, but maybe it is there already and I missed it.
Their builds would be based on crafting and exploration.

2. Bersekers would have that elixir quest possible to finish and by drinking that you would be more hairy - like a side effect of mana potions. Also those seedlings wouldn't be just on the ground in gardens, but it would grow through the whole village, making it more interesting/fantasy to look at (sort of Morrowind like mushrooms).
Their builds would be based on rangers kind of gameplay with better bows (missing legendary bow, crafting arrows and pulling the string animation for heavy shoot), and also druids (missing in the game).

3. Clerics would have builds based on palladins with heavy armor/some spells with possibility to play a real pure mage.
By siding with them it would continue in Hort - I sided with them, but it didn't open the gate for me there.
Also I wish they would use capes and cloaks more - like that one in the main picture. I wish they would be more cool, not just stupid most of the time - more gothic like palladins.

4. it would be possible to join separatists Albs, and by doing that obtain white color like before
and I wish drinking elex would have real consequences in the game and changing color aswell.

And I wouldn't mind survival mode added instead of fighting with skull mobs all the time.
I wish skull mobs would be in dangerous areas mainly, so exploration would be possible even with not so good equipment.

5. I wish stats would be according to that gameplay, so it wouldn't happen that I would need 70 strength for ranged weapon, but for heavy weapon, so it would be more logical. But I guess that would require more complicated system and I'm not an expert, I just wish it would be smooth/logic to create such build and maintain the fun of your own build/gameplay, even at the cost of damage - like survival mode with less damage, skull mode with less survivability or something like that.