I agree with you because of some half-formed theories of mine too.....
1. Kuroba, it could linked to the infamous tune, for the feathers of a crow are black, and yes, he was mentioned subtly in the series too.
2. He is linked to Vermouth directly from the past, and Vermouth being his favourite could be because she was his direct disciple.
3. He has the reasons and power to connect with people and influence them, like the case with Kudou Yuusaku.
4. I think as a distinguished writer, Aoyama-san might just want to link his creations together, to complete both Conan and Kaito together in one ending would be a great finishing touch.
Though, personally, I wish there was some other explanation or reasons for him to do so, because for me, both Kaito and Touichi are protagonists instead of antagonists in my eyes.
