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Law was so sure himself that he couldn't defeat Doflamingo in a fight that his whole damn plan to destroy Doflamingo was being killed by him but dying sure that Kaido was going to kill Doflamingo later for his failure to keep producing Smile.
Law's raison d'être was killing Doflamingo and the only way he could think of succeeding in doing it involved dying against him and relying on someone stronger to do it for him.
Law never even dreamed on defeating Doflamingo, not even with Luffy's help, and if he is that sure he is completely below Doflamingo's league, I can't see how anyone else could argue otherwise .
And that is why I would love to see current Luffy to be as strong as Doflamingo. I get feeling that this might be the case when Luffy heals from his fight with Katakuri.
That would put Luffy closer to being as stong as an Admiral- if Luffy in Gear Fourth is as strong as Doflamingo and he is getting closer to become as strong as Katakuri he will be facing an Admiral after Kaido arc.
Fight against Kaido should be like Oz vs Strawhats but Kaido would be Oz and in place of Strawhats we would have Supernovas.
Then next real 1 vs 1 that I would love to see would be Kizaru vs Luffy. Luffy promised that Strawhats will beat Kizaru one day and I expect that Oda has Holy Land arc planned where Luffy will face Kizaru and others will face Sentomaru and Kuma.
When Kaido will be beaten, Big Mom is weakened(her forces) it might be that world will become more chaotic and Marines might also make their move. It would be best moment for Dragon Revolutionary Army to try and attack Holy Land.
Laws immortality might come into play after Wano. Doflamingo wanted to become immortal so he can get his hands on something that is located in Holy Land.
Law most likely will be dying in next arc and he will pass this power of immortality onto somebody for that person to try and use it for Treasure Doflamingo mentioned. Oda did not hinted all of it for nothing through Doflamingo.
Also we have gotten some build up for CP0 and Revolutionaries over last few arcs. Oda also had his notebook "Dragon" next to "Big Mom" notebook. So after Wano I expect arc taking place in Holy Land with CP0 and Revos being in center. CP0 is strong enough for BB pirates to back off(weakened by fighting with Revos but still powerhouses) so that would make them great enemies for Strawhats after Wano.