If Luffy's DF is indeed Special, i believe it combines Three Concepts of Haki (CoA, CoO & CoC)
Oda didn't focus on One Type of Haki this Arc (For example in Skypiea, only Mantra was Highlighted)
This Time, he spent a lot of time Explaining CoA & then switched completely to CoC & before Luffy's Defeat, we saw Kaidou activating Future Sight ... etc
The Idea here is that Luffy tried all of his Three Haki Types & still couldn't Best Kaidou
This is similar to Doflamingo Fight, where it was clearly implied that Luffy's G2 had Speed but lacked Power while G3 was opposite
So he revealed Gear 4 which was Both Strong, Tanky & Fast
This is why i believe Luffy's Next Power-Up should represent Haki System as a whole, but Used Infinitely
How do you have Infinite Reserves of Haki? It's by "Absorbing" Energy around you, similar to Sage Mode from Naruto or Spirit Bomb from DB
Rubber = Ability to Bounce Back
It's represented in how Luffy always comes back against Villains
I believe that Luffy's Next Power-Up is to Bounce Back Attacks
Like imagine when Kaidou attacks him, Luffy absorbs the Attack & aims it back at Kaidou, cuz that's what Rubber does
Similar to when he uses Gomu Gomu No Balloon to return Canon Shots
This Luffy Ability will make him able to Fight Non-Stop & get stronger the more he Fights
Similar to Metal Bat from One-Punch Man