Finished Mohrta.
Highly recommend.

>Difficulty
Low to medium. It's a Scumhead game. Enemy archetypes line up fairly closely with regular doom enemies. Lots of variety though. The enemies are also designed with your dodge dash in mind so you will have to use it.

>Level design
"Open Linear"
As in, the levels are fairly open, but also have linear progression and lead you very effectively. I think I only got truly lost like once near the end.

>Art design
Look, this is the selling point. You know it I know it everybody knows it.

This is what Mohrta is. It's an overwise pretty normal "modded doom" type game with god tier art design.

Also has a surprising amount of story. Multiple endings, even.