Right now sylvari are the only things in the game that have bioluminescent parts. I was thinking it would be great to see creature designs incorporate this as well.
And beyond cosmetic reasons to add it you could have enemies that change behavior and tactics based on the day/night cycle. Especially if the bioluminent markings were distinctively different from its normal markings this would serve as a visual cue as to what tactics it will use and how you will have to approach the fight.
Other creatures could use it as a flight or fight signal warning of stance changes between aggressive and defensive fighting.
Overall it could be used in New enemy types to add depth
You can’t spell Slaughter without Laughter