So, I was watching a video by a well-known Zelda Theorist on YouTube named Dr. Wily. He postulated that the Mushroom Kingdom and Hyrule could not only be in the same gaming universe but could be sister countries.
He used the example of games like Link's Awakening, Oracle of Ages and Oracle of Seasons to address the canon of Mario enemies being present in LoZ games. The Chain Chomp seen in the Mario games is present in the aforementioned games, and Goombas also make an appearance. He did not count things like easter eggs and references, but actual in-game appearances of these character enemies.
I'm not going to say that I am in support of this Zelda theory, but I did find it interesting from a philosophical point of view. The only thing threading these two game worlds together are the elements of mysticism that are attached to both worlds.
Still, I think the undoing of Dr. Wily's theory is that LoZ and Super Mario have two different tones that could never mesh well, even in a crossover game. LoZ is mature and gritty, even the cartoony Wind Waker has serious elements to it, and perhaps the grittiest of the Mario games, Luigi's Mansion, was still too lighthearted to be considered mature. Another thing to mention is that LoZ, while indeed having anthropomorphic populations in the form of the Gorons, Zoras, Rito, the Oocca, and so on, also have an amass of human-like beings; the Hylians, the Sheikah, the Gerudo, and the Humans who are termed as Hyruleans, and yes, while Link in Twilight Princess is referred to as a human by the Gorons, he is actually a Hylian, and is not really human. Humans in LoZ are marked by having round ears while Hylians have pointy or sharp ears. While in the Mario games, the only humans to appear in-game are Professor E. Gadd, Mario and Luigi, Rosalina, Princess Peach, and Princess Daisy. Outside of these characters, the majority of the residents of the Mario universe are nonhuman. So, unless they are on different continents to each other, they would have to be parallel or alternate worlds to each other.
So, thoughts? I'll create a poll just to get votes on this, but I think most of you will vote against this theory, even if it's fun to ponder.