Ambition Hardening has been shown to restrict create/become Devil Fruit users. While control users are seemingly unaffected by it, it stands to reason they would have some limitations using it as well. The point of Luffy discovering how to vulcanize himself would lift some of the restrictions that Ambition hardening gives, and would be especially necessary if there were those out there that could nullify Ambition (I believe 100% that in the Arc where Vegapunk is finally introduced, we will see some sort of Null-Ambition device. If there's Seastone for Devil Fruit powers, there's definitely something to block Ambition, too.)
Also, he would need this to avoid being flat out melted by Akainu. Kuzan's cold wrecked him pretty badly, and I'm sure Ace's flames would have too, had they fought as adversaries. Unless there's going to be a Sun Logia, which is doubtful as there's already a Light one, Akainu has the hottest and one of the deadlist DFs in the One Piece world (I really wish they'd give this planet a name so we could just refer to it as such). Vulcanization would improve his chances against burns if his Ambition fails to protect him. The question of "How" he will do this process remains to be seen, as it generally requires being treated by sulphur in real life. Maybe a Sulphur-based DF user will appear?
I can see other users improving their abilities in creative ways like this. Crocodile and Eneru could team up and give Croc Fulgurite armor, or Kuzan could accidentally create Obsidian armor for Akainu, which begs the question if that's actually what happened during their fight. Akainu hasn't been fully shown, maybe part of his body is obsidian now? He should go fight some White Walkers…