How would combining different types of Haki work?
I've had this head canon for a while that Voice of All Things, Shanks' "observation killing" and other unexplained feats might be a combination of Observation and Conqueror's Haki. If the function of Conqueror's Haki is to assert, or project your own will onto others, and Observation Haki is to respond to external stimuli - then combining them, could it be possible to control the stimuli someone else senses through the assertion/projection of your own will?
If you want them to hear your voice telepathically, like the Voice of All Things is shown to be, then maybe with the combination of these two types of Haki, you could make them hear your voice, just because you willed it so. I think the same could be said for Shanks negating Observation Haki and Yasopp live-streaming his Observation Haki to Usopp in Film Red (although it's not canon, Oda was still involved) - I think that suits the definition we've set for this pretty well.
But I haven't thought about the Observation & Armament combo or the Armament & Conqueror's as much - black blades did come into mind as a potential application of combining Armament and Conqueror's, and but to my knowledge, I can't find an example that I can say fits what the combination of Observation & Armament might be, so if you have any ideas, I'd love to hear them.