If it's a contest that allows original designs, if it's any good then the whole "well, they can't judge original design for accuracy" shouldn't matter. Reproduction costumes should be scored for the accuracy and the execution of workmanship, and originals should be scored for intention and execution of design and...the execution of workmanship.
Since you're basing it off of something that exists in media, you're in a grey area. What you should do is include your reasons for deviating away from the character design. I'm not familiar with Diablo, but I know a lot of in game design vary widely from the concept art. Maybe with your ref pics include in game pics, concept art, maybe sketches of your own, and some bullet points outlining your rationale.
If I may brag about my own experience, my Madame Red was quite different from what the character wore in the manga/anime, and it did very well in a masquerade. The changes I made were in the interest of historical accuracy, and I made sure to show to the judges with my documentation that I was very familiar with fashion from 1888 (the year of the Jack the Ripper murders, which happened at the beginning of the series).
your mileage may vary, but always, always make deliberate design choices and explain your choices!
Fluttershy (MLP), Lady Loki (Thor comics)
Joffery Baratheon (Game of Thrones)
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