First, thanks everyone for making my first blog popular and commenting on it. This will be blog number 2 so hopefully it will do just as well. So, I've decided to leave the chapter reviews up to the people who are already good at it. For now...
Here's the deal, I've seen a few people point out that everyone is gonna have Haki in the New World and that your level of mastery of it will ultimately decide how strong you are. It'd be stupid if it just stayed almost exactly the same for the rest of the series so here are some ideas about how higher levels of mastery might alter it's use.
Color of Observation: So far we've seen this to mostly be a tool to predict and dodge attacks, but also to sense everyone around you like Enel, Aisa, and Coby. Lastly, Luffy and Rayleigh have used it to measure strength of those around them.
Some ideas I had for this one's advancement include clairvoyance (beyond just predicting attacks) and increasing your accuracy of long range attacks (without needing to see them.)
Color of Arms: Currently this can be used to harden your body, push others with invisible force (Sentomaru), imbue innanimate objects with power (like arrows on Amazon Lily), and hit Logia users.
Ideas I've had for this one's advancement include walking on water or up walls, creating shockwaves that cause small earthquakes or tidal waves (superpowered version of Sentomaru's attack), or empowering nonweapons like ships so they might survive an otherwise devastating attack.
Color of the Supreme King: Only thing this has done so far is knock weak-willed people out although it's implied that it helped Luffy tame the Kraken.
My idea for this one is to dominate the wills of others and essentially control or influence weak-willed others (sort of like the Jedi Mind trick. These aren't the pirates you're looking for)
Anyway, these were my ideas. Lemme know if you like or dislike them. Come up with and share your own ideas. I completely left out how these would enhance Devil Fruit users abilities so you can bring those up too and add your own. Just have fun with it and let your imaginations run wild.