Since i just made a suggestion about Haki Colors, why not keep going.
Armament Haki, in the manga it makes you more durable and enhances your attacks and makes you able to hit logias !
In the game, let's make it grindy ; You'll have to train your haki to reach new tiers of mastery, maybe 3 Tiers Max, there could be few ways to train haki im not too sure. Maybe taking and dealing damage would make a hidden stat go up and once you reach a treshold it your haki evolves in a flashy animation !
To not make it too OverPowered, Haki could have a duration/durability before it breaks or both making you unable to use it until you recover.
Each tier giving a buff, and increasing the duration of your Haki;
T1 Buso : You barely master it and can just defend yourself with it making you take XX% less damage while activated. I'm leaving XX because i am terrible at balancing, i'll let you guys figure that out .
T2 Buso : You can now reliably use your haki during combat, able to enhance your defense further and your attacks ! Take XX% less damage and deal XX% more damage while activated ! You are now also able to hit non physical ennemies ! (Logia and maybe special mobs making haki a requisite to farm in certain areas).
T3 Buso : You are a master at using buso, a cannonball wouldnt leave a scratch on you, how scary... You now take XX% less damage and deal XX% more damage ! Oh, what is this ? You're fancy and colorful
(T3 gives a color to your haki go check my other suggestion for more info)
Thanks for reading this far, i dont have a clear idea for Observation Haki that wouldnt make it absolutely busted i'll make another suggestion later !
EDIT : As for the obtainment method for buso, i think a simple quest is kind of lame why not make it so you need to risk your life to awaken it ? Get into dangerous situation during capture events or even pvp with a certain % chance to awaken, and if you're unlucky just make the % go up each time the player risks their lives so they get better chances to awaken it. It would encourage people seeking pvp events. Plus since pvp events arent that frequent i dont think people would find a way to cheese that... but we're never too sure lol :v