It's not a bad theory. The whole immigration thing would parallel with that origin story of what town/city Marco resided in. Language can be disregarded since I think that's just a show thing. It'd be annoying to have subtitles the entire time. I'm not sure about the magical part since it doesn't seem all mewmans can control magic. It seems to be something that the people of that planet/universe have. The Johansens from a different kingdom don't seem to have magical powers although they resemble humans, but there are demons like Tom, pony creatures like Ponyhead, and those weird demonicism cult dudes that all can use magic, yet aren't human like (maybe Tom).
It's far fetched so I don't believe it