Yamao got some actual points this chapter with her flashback. Shimotsuki Ushimaru could play the role of her mentor figure if there is another flashback that follows up. It was obvious that Oden couldn't be her mentor figure and I said a while back that Yamao that it's essential for another figure to appear if she's to be a serious candidate, so points there. That being said, it doesn't seem like she'll have an extensive flashback based on the introduction so...
Either way, the ground she gained there, is heavily outweighed by the stuff with Kaido. People used the excuse "Kaido's just going easy on her". It's now confirmed that he's not. He's in hybird form genuinely trying to kill her...but can't.... The strength she displayed easily surpasses anything Sanji can dream of accomplishing. I said it before but this just makes it totally undeniable that she'd destroy the M3 if she were to join.
People say, "Oh but Jinbe, Robin and Franky were strong too." But the situations aren't remotely the same.
Robin's greatest feat in Alabasta was beating Pell. And his greatest feat (excluding asspull nukes), was beating some fodder. When it was time for her to fight the big bad she literally got no diffed. The fight lasted about 5 seconds and Crocodile beat her in one hit. She was strong, but didn't do anything Zoro and Sanji weren't easily capable of either.
Then we have Franky. He was able to fight evenly with Luffy and was shown to be a force to be reckoned with too. But again, when it came time for him to fight Lucci, he also no diffed. The fight lasted seconds and Franky was out. When it came to the end of arc fights, the M3 got power ups and fought the 3 strongest CP9 agents, whilst Franky was struggling with Fukuro the second weakest. He was clearly beneath them.
With Jinbe, Oda delayed him joining the crew for close to 10 years until Zoro and Sanji got power ups. When the final battles came around, Zoro and Sanji got a calamity whilst Jinbe fought a Tobi Roppo. Oda didn't even settle with him fighting Jack in a fishman battle. Instead he dropped Jinbe's opponent down to Tobi Roppo level alongside Franky and Robin.
All 3 of them may have been strong, but when the end of the arc came before they joined Oda ensured that they didn't disrupt the hierarchy in the crew. Luffy/Zoro/Sanji will always be the 3 strongest straw hats, no matter who joins. I greatly struggle to see any plausible way in which Sanji can become strong enough to use advanced CoC attacks and clash with a blood lusted hybrid Kaido. Her power level just has no place in the crew dynamics, so it's a really huge blow against her.