This may be a bit of a stretch, but I've got this crazy theory that he can fly.
If we zoom in on this manga panel we can observe that he appears to be in the air without his feet touching the ground. How is this possible? Well, from what I've gathered he's somehow obtaining thrust by flapping his wings which is seemingly giving him some sort of momentary flight capabilities. From my extensive research, birds may also occasionally use gravity itself to provide forward thrust as well; which explains how he's magically moving forward without the use of his feet. I think King may have a second devil fruit.
Now before you say I'm jumping the gun, let's look at some rather empirical evidence.
As we can see here, Blackbeard's jolly roger has three skulls, one for each of his Devil Fruits; the Yami Yami no Mi, Gura Gura no Mi, and the Inu Inu no Mi, Model: Cerberus. And as we can see in the image above, King's overcoat displays two skulls. Coincidence? I think not.
So assuming this theory is correct and he happens to have two Devil Fruits, the first being the Batto Batto no Mi, what could the second one be? Let's look back at some of the known context clues:
• He can move through the use of some unseen force
• He's producing heat energy
• He's in the sky
It's very likely he has some sort of awakened sun devil fruit.
If my theories are correct here's what I imagine King's match-up at the end of Wano could possibly look like.