Oh god, are people unironically joining the Kizaru for nakama train now? This is why people make fun of us.
If you had told me Aokiji was going to join the Blackbeard Pirates Post-Marineford I would have laughed so hard. That totally shocked me when it happened.
Oda's crazy, you never really know what he'll come up with. Look at Cross Guild.
Who the hell would have predicted Buggy becoming a Yonkou after accidentally being labelled as the leader of a criminal organization that assassinates Marines???
It's a longshot for Kizaru to join, yeah, but, we don't know. We KNOW Kizaru doesn't like what he's doing. But, he called himself a Corporate Slave, because he knows the Navy can't successfully overthrow the WG. If Kizaru gets proof here that the Straw Hats can actually cause a Gorosei problems, he could change sides.
IE; if Kizaru knew joining the Straw Hats in an alliance could ACTUALLY save Vegapunk's life, and that they could protect him, I think Kizaru would turn on Saturn.
If Kizaru did make a temporary alliance to save Vegapunk here, that could lead to him joining. That would need to be the first step to get him in the door for serious conversation. Until that, it's all baseless clown speculation.