Rumble would likely not encounter the Human Graduates from Heimerdinger's Academy. They would never find Bandle City.
Rumble had an issue with creating things for Human Use. He didn't see the point in it.
Rumble had the perception that Humans looked down upon Yordles. I have no idea how this perception can develop with Yordles being such a rare encounter for most Humans.
Rumble preferred making gadgets, which is why he was encouraged to join Heimerdinger. I believe this suggests a desire for machinery was not a commonplace trait among Yordles.
While Rumble does not wish to share his creations for Human Use, he does want to share that his creations exist. This desire for acknowledgement is crucial for Rumble to have any chance to manifest in the Human Realm.
Assuming Rumble could manifest, it would likely be in Piltover. While I agree that his original personality does bear quite the resemblance to the children of the Zaun slums, I don't see why a Yordle would manifest from "Angst". If he did manifest, it would likely be around the love of mechanized creation, which I think is more prevalent in Piltover. He would be that Yordle that appears for every "Science Fair" and then promptly vanishes afterwards. The whole concept of science presentations feels more natural for Piltover than Zaun.
For a more consistent state of manifestation, Rumble would also need the love of fixing machinery added to his lore. If that was part of his character, he could manifest as some sort of mechanic that helped make machines work. It may also break him away from Piltover, since mechanics are probably in equal demand in Zaun. He could still remain antisocial (as I'm sure Heimerdinger is).
