I would rephrase the question into this: What makes an open-world fun?
Y'see open world games are dime a dozen nowadays, and frankly, just a selected few are actually worth bothering.
Meself found three big recurring errors in todays OW games flood:
Realistic, yet still boring (AC: Origins, etc).
Wonderful to look at, but great looks can get very natural in the eyes in about … hmmm… 10 minutes.
Full of stuff, yet still boring (Witcher 3, etc).
I just hate when my character is the only capable fixer of everyone's personal problems. Living-breathing my ass.
Diverse, yet still boring (Horizon, etc).
You've got the obligatory snowy part, the jungle part, the lava part, the swamp part, etc. Videogame world design 101.
IMHO the best OW games have just one thing that stands head and shoulders above all the benefits of the aforementioned world designs: character.
Sadly, this is where Elex is lacking as well.
Oh, and one thing: before I'm gonna be crucified in a nice flame war, please mind that I criticize the world design and not the game.