Luffy has been chanting Kaido's name from Punk Hazard. If he does not play a major role in Kaido's death and doesn't defeat him one on one, he will look like a joke. And I doubt Oda will let that happen.
ZKK is a solid theory wirh lots of hints and foreshadowing, its upon Oda how does he wants to execute it without diminishing Luffy's importance or even wants to execute it.
I already posted this on another thread, but here is a theory:
1. Kaido gets knocked out by Luffy in a solo fight after a group bout and falls into the seas. With no one to hold Onigashima up, Momo repeats the hour of legend by keeping the island up while the flower capital is evacuated
2. Big Mom defeats Law and Kidd, but not before being dealt a fatal blow. With Kaido seemingly gone, she then decides to overtake Wano as the pirate hive. At the same time BMP finally makes it to Wano and launches an attack.
3. The PNM alliance confronts BMP.
4. Luffy gets juiced up by the Zou elixir (only if he is too injured) and confronts Big Mom. They have a grueling fight which ends with Luffy decisively beating Linlin in a solo fight and turning her permanently into Olin. Meanwhile, PNM alliance also defeats BMP.
5. With all the threats seemingly gone and the dawn nearing, PNM appears to be the winners...but then Momo is attacked by a Boro Breath and loses control of the island.
6. The island's control is overtaken by the returning Kaido in dragon form, who decides, with his crew and masterplan gone, he is gonna commit suicide now and take the whole world with him. Kaido lifts Onigashima over 20,000 meters in the air before letting it drop (basically pulling an Ultron).
7. The PNM puts up together a plan: Onigashima has to be destroyed before it reaches Wano and Kaido has to be killed so that he cannot repeat the process with another island.
8. Through an intricate series of plan, the PNM work together, with Luffy unleashing a gigantic King Kong Bazooka to destroy Onigashima to pieces.
9. With the dawn soon to be breaking, Marco launches Zoro towards Kaido for the final time. Kaido unleashes a gargantuan Boro Breath, but Zoro splits it, channels Asura, and finally kills Kaido atop Wano as the dawn breaks.
This way, Luffy took out BM and an island.
Zoro killed Kaido.
Neither one got overshadowed by the other.