We need more haki x logia interactions. Logias can turn into an element, and having that element be buffed by the user's haki could be a very interesting concept.
For instance, remember how Akainu's magma could burn fire?
Imagine if this was a custom technique Akainu evolved with Armament to buff his magma.
Another example could be Aramaki with his fireproof forest:
Imagine his Flower Power Flight was a result of him buffing his flower with haki:
So basically bending physics using haki. I could see Kizaru creating laser shields and hard light constructs, Kuzan hardening his ice to diamond levels of toughness etc.
Another example is Future Sight x Body Morphing.
Where mastered COO can allow logias to auto-evade attacks. I wouldn't be surprised if this was already a thing.
Another possibility is using ACOC to
evolve an element after mastering it.
For example this is Big Chill from Ben 10, who uses ice powers:
Meanwhile Ultimate Big Chill can breath fire so cold that it burns AKA freezing fire.
Swampfire is made of plant matter and can shoot flames.
Ult. Swampfire is made of petrified wood and shoots hotter blue flames.
So imagine say Akainu using magma that he evolved to be hotter or more explosive.
Lastly, transformations.
These could be personalized to the user, like Enel's godly form and GB's tree form. I fully expect the C3 to showcase versions of these possibly based on their animal motifs.