Does that guarantee him being the second most relevant factor in this arc besides Luffy?
He's been set up to do things hasn't he? That's how that works right? You give a chaacyer focus to set up what they'll do later in the arc.
There's set up with him training Momonosuke and whatever is going on with Hiyori.
Theres set up with him to help with the figting against kaido
There's still set up for him to learn to cut fire so he will fight King
There's set up for him to create black blades or learn of the creation at Ryuma’s temple
There's set up dor him to kill Orochi or at least let Hiyori kill him like she said she wanted to.
There's set up for him to fight a strong female swordsman ie smoothie
There's set up for him to have some involvement with this Shimotsuki clan that keeps appearing.
Theres set up for him to get more sword like Nidai Kitetsu and such.
None of his set up would exists if the arc wasn't literally focusing on Zoro so much up to now.
Don't get me wrong, Maybe some of these won't pan out like how the "gathering samurai" thing didn't pan out. Since Oda can change his mind about the direction of the story but until we see on panel that Oda has changed the direction of the story, then all this has still been set up.
The question is, Do these things seem relevant to you? If Zoro kills orochi, cuts kaido and 1v1 king and defeats him, Does that make him the second most relevant character in the arc? Well I would assume so. That is unless like some Admirals appear and do some ridiculous shit during the war or shanks shows up and does some ridiculous shit. Or Like you seem to want, maybe kid is revealed to admiral level and like Solos big mom or something. Who knows. But as of now, I don't see anyone else outside of Luffy being more useful here. Maybe law has come up wth a master plan to take out kaido but then plans fail in one piece so no that wouldn't make law the second most relevant