Hello, so i had this idea about Armament Haki and Observation haki.
I think it would be pretty cool if each player gets a random color outline on their Armament Haki once mastered;
Same thing for Observation Haki, kinda like Katakuri's future sight (Advanced Observation Haki). Some techniques could also get infused by Armament Haki and change the visual effect of said attack with black coloring (main color of haki) + their random color.
Another thing you could do is add a new product similar to the Hair color product already in store ! It would let player change their haki colors ! Probably make it so it actually let them choose or else it'd be a rip off lol. I'm attaching exemples to show what i mean.