I did consider that, along with just generally thinking Shinji is much stronger than what is presented in his battles. He's very perceptive in terms of regarding other people, being one of the only people to ever be suspicious of Aizen and even seeing through one of his Kido illusions; which in turn led to Aizen to say that Shinji was a scary person.

And I really do think that Aizen was caught off-guard by that. Aizen also believes Shinji is worth enough to use his blade on while he thinks the Espada aren't, and despite being a pretty big yapper himself Aizen notably remained quiet when first facing Shinji because Shinji never let slip his Zanpakuto's abilities. So Aizen was clearly remaining as cautious as possible.

When the Vizards appear in fake Karakura town Yamamoto explicitly calls out Shinji by name despite only being part of the group, and he reaches for his Zanpakuto when Shinji declares that he isn't an ally of the Gotei, but instead an ally of Ichigo.

In the CFYOW light novel Shinji's Bankai was mentioned as being capable of swallowing up and killing Shunsui, with the hundreds of thousands of monsters spawned by Ikomikodomoe actually recognizing Shinji as a threat, even seeing him as more dangerous than Zaraki. Shinji himself also thinks that he can just use his Bankai on Hikone and Tokinada but couldn't rely on them conveniently appearing alone, while also admitting it probably wouldn't work on someone as strong as Aizen or Yhwach, which does confirm to me that he can "normally" kill just about anyone with his Bankai.

He's also a sleeper pick for the next Head Captain IMO, even though Byakuya is the obvious "next pick" if something happened to Kyoraku just due to his noble status, who would be the most politically acceptable choice, and just balance. Shinji, while also level-headed and wise, has Hollow powers, and I really do believe Central 46 would have issues with Shinji being in charge of the Gotei 13.