I know it was stated once during the Skylar remodel that Mourn was under consideration for one as well in the future and I do hope that is still the case because compared to the others he is the lowest quality looking hero on the roster IMO.
Everything from his vines, bark, and model look very dated and not up to par with the rest of the cast. He is the only hero thus far that feels underwelming visually wise for me. I know there was a changing of hands on the art team during his design process so Im sure that might have effected it.
I personally feel it should lean more into his haunted and dark side of his persona, have skulls adorning his body, hanging from vines, and the protrusions from his body. Maybe even have spirits wisping about him or even moving without and within him. I also think his vine arm needs to be focal point of his design, I would suggest making it an actual animated mass of vines that move. His animations I would also suggest updating and revamping as well along with his visual design.
I like Mourn as a character and enjoy playing him, just compared to the the others he feels lacking visually and would like to see him brought up to par with the rest of the roster.