If I’m trying to salvage the stuff I put on Twitter, I’d be remiss if I didn’t take my theories about the Mushroom Kingdom:
In the Mushroom Kingdom, it appears at least, that permanent death has been defeated (presumably by the 1-Up mushrooms). Are Boos, then, the ghosts of those who died before the breakthrough, or are they ghosts by choice?
They do, as a general rule, tend to reside in abandoned homes and castles. Could these be the remnants of their pre-fungus society?
I also have reason to suspect that pre-fungus society was made up to Koopas and related species, such as the Yoshis or other dino types such as Rex from Mario World. These were the dominant people of that world pre-fungus.
There is controvery over whether Mario and Luigi are actuall from New York or if they are Mushroom Kingdom natives. I feel that their foreign status can be the only correct reading. They are definitely New Yorkers.
I suspect that there exist dimensional anomaly, likely accessed via the pipes throughout the Kingdom which connect the once-Koopa-ruled planet with other dimensions, including the alternate world ruled by humans which the Mario Bros hail from.
In fact, it is almost certain that all humans (and indeed, the Kong family of apes) in the Mushroom Kingdom come from contamination via the anomaly.
Likely, humans had been inadvertently wandering into the Koopa World for centuries via the anomaly. Given that they successfully built a sustainable population there, it seems the Koopoid races treated the accidental visitors with some kindness.
That peace presumably only lasted until the fungus, though. I don’t know if the fungus originated in another dimension or if it is native mushrooms mutated by the strange energies of the anomaly, but it seems to be the most important thing to ever come to that world.
Suddenly there came a host of magic in mushroom form. Mushrooms that could change one’s size. Mushrooms that could give a car a boost. Even Goombas! A sentient species of mushroom. But the 1-Up was the most world-changing.
It does seem that in the modern age the Koopas and allies have access to the live-giving fungus (look how often Bowser and his kin come back from the dead), but perhaps they did not take to it originally. If they opposed the use of the 1-Up, they doomed themselves.
An enemy who can not die can defeat an enemy that can. They only need to outlast them. So the humans who embraced the 1-Up, likely the founders of the Toadstool dynasty, won the struggle for planetary dominance.
Now you may say that the Toads are not humans, they are fungus people. Yes, they are. Now. I suspect that the 1-Up works as follows: your consciousness is backed up and stored in the fungal network. When you die, a new body is grown from the fungus.
The process is likely not perfect. Every time one dies, their regrown body is likely slightly different, slightly more mushroom than before. This is also probably why Mario and Luigi look so much more cartoony than regular humans. They have died too often.
Peach, being royalty, is probably given special care in the regrowth process. The recent powerup that can turn Toadette into a Peach doppelganger proves the fungus can specilize in her form.
Potential point to ponder: do Shy Guys keep themselves hermetically sealed to resist exposure to the spores? Are they a sect left over from the era of the struggle?
Likely the bricks that are so ubiquitous in the Mushroom Kingdom (those which are not people transformed by Koopa magic anyway) are the remains of the great Koopa cities.
So what I am saying is I think Boos are ancient dead Koopa people.
We have to accept that Bob-Ombs are likely artificially intelligent weaponry that originates from these ancient wars. Left running in the thousands of years that follow, they have gained a culture all their own.
The “shells” worn by Koopas are not biological. They are technology. They can take them off like clothes. They can ride around on them. They can use them as weapons.
The ancient Koopoids are also likely the designers of clouds that can be walked on (or even driven around) and the vast pipe system that allows world travel with such ease. They must have been an amazing people.