Sorry to bring this back up, I know it hasn't been posted in for quite a while. I just had a theory I wanted to share that I've been thinking of for a while now, and this is the best place I can think of to share it :p Anyways...
So my theory involves a little bit of continuity between games; particularly Super Mario Sunshine and Galaxy. Maybe some of you have thought of this one before, but I didn't see it posted so I thought I'd post my version. I won't say what it is until the end of the theory to inspire you to read it and come to realize it yourself as we go along. So here we go!
So to start, in Super Mario Sunshine we are introduced to a brand new villain (or so we think) named SHADOW MARIO.
So he starts out as a bit of an enigma, but throughout the game we learn a little more about him:
-He looks exactly the same as Mario, except he is completely shadowed in a blue tint.
-He has a high-pitched voice; even higher than Mario's Yahoo's
-He is evil, kidnaps the princess and vandalizes the island, framing Mario for the work.
-He is usually seen carrying a device that looks like a giant brush.
-SPOILER AERT: It ends up being Bowser Jr. in dusguise.
-Apparently his "Magic brush" gave him the ability to shape-shift into a Mario clone, as well as several other abilities such as the ability to paint portals and have infinite goopy paints.
Skip ahead a bit to Super Mario Galaxy. Bowser Junior appears in that game, sans Magic Brush or any sort of Mario Disguies (in fact, he is never seen in disguise again in a main Mario game). However, there is another Mario lookalike, with some suspicious similarities to our old friend. Enter COSMIC MARIO
Like Shadow Mario, Cosmic Mario is a bit mysterious, and unfortunately, we don't learn too much about his origins or intentions. We can only assume based on our few encounters with him...
-He looks exactly like Mario, except he is totally shadowed in a blue tint.
-He has a high pitched voice, even higher than Mario's Yahoo's
-He is a jerk, but not necessarily evil. He would happily nab a star from you, but he doesn't kidnap innocent princesses or seem to do bidding for any higher power.
-He loves to race and seems obsessed with beating Mario on the track.
-He seems cocky, in his laughs and gestures.
-Nothing of him is ever shown below his hazy cosmic surface; it is unknown how he gets his powers, or if they're natural or augmented.
-Bowser Junior has an alibi this time, as he appears elsewhere during the events of Galaxy. Also he does not have the magic brush to transform himself this time. For all intents and purposes, we can assume Bowser Junior is NOT Cosmic Mario.
But you may be saying "So what? Nintendo probably just needed a quick easy idea for someone for Mario to race in Galaxy so they used a clone. He's probably blue because that's the color of deep space in the game."
Well, that probably true. But there is one thing that could serve as a very solid link between Super Mario Sunshine and Super Mario Galaxy, and between Shadow and Cosmic Mario. And he comes in the form of an old friend//enemy/frienemy from Super Mario Sunshine: IL PIANTISSAMO
Wait, what? Who? You know, that guy who appeared in like three missions, to challenge you to a race. Although at first glance, he seems to be a little out of sync with these other two characters, if we look into it a little more we might see a few key traits that seem very interesting for this analysis.
-He appears to be a human being of African Descent (finally, some racial diversity in Mario games).
-He wears a very racially insensitive outfit consisting of a Pianta mask, and gloves and boots resembling pianta hands and feet.
-He clearly has NO ISSUE DISGUISING HIMSELF. This is probably for his selfish benefit, as an average human would seem fast to the typically bulky, slow Pianta people.
-He's a bit of a jerk, but not necessarily evil. He'd hog an all-important shine sprite from you, but he doesn't seem to serve a higher evil power, i.e. Bowser.
-He loves to race and seems obsessed with defeating Mario foot-to-foot.
-He seems cocky in his laughs and gestures.
So if you take the non-physical aspects of Il Piantissimo, we see they match up pretty much perfectly with those of Cosmic Mario; they're both cocky jerks who love racing, and obsess over proving they can outrun Mario. But you might be thinking "No way, Il Piantissimo looks nothing like Mario. Even with the best props Party City has to offer, he couldn't get the shape as on-point as Cosmic Mario does. Besides how would he ever get into space?"
Well, you'd be right. Piantissimo simply couldn't replicate Mario's form (he couldn't even make a convincing Pianta). And to go from a tropical Island to outer space? Forget it. What kind of marvelous technological masterpiece would Pianissimo need tot make himself look just like Mario despite their hugely different shapes, AND make him able to travel long distances in the blink of an eve?
Wait, hold on a second....
In the final battle of Mario Sunshine, after you flip Bowser and son's giant floating hot tub, Bowser Jr. Seems to have lost his Magic Brush....
Where could it be? It must have fallen to some random location on the island below. But there's no way anyone would come across--
OH MY GOD!
That's right! The Ending screen confirms it, the brush just happens to fall right onto Gelato Beach, where our old friend Piantissimo just happens to stumble upon it. Now, it's like the stars aligning! Sometime between Sunshine and Galaxy, this must have happened:
-Piantissimo finds the brush and learns to harness its powers.
-He uses its shape-shifting ability to paint himself the shape of Mario. Probably to mock or intimidate him.
-He uses the brush's power to create portals to track Mario down into deep space.
-Piantissimo is a sore loser; he follows Mario to challenge him to rematches.
-He now has the same cocky, jerkish personality, but the distinct look of out favorite Italian plumber (with a dark blue tint).
Piantissimo IS Cosmic Mario! They are one in the same.
There are a few discrepancies to this idea, but I would say they are mostly either negligable, or could be explained fairly easily.
1. Shadow Mario looks like he's made of water, but Cosmic Mario looks like he's made of deep space matter. If it they both used the magic brush shouldn't they look the same?
-Not necessarily; the magic brush was shown to be able to produce paints with many different colors and properties. Shadow Mario probably chose the water look because he was on an island surrounded by water. Cosmic Mario probably went with Space theme because he was surrounded by outer space. It's the theme of the game.
2. But Shadow Mario is always shown with his Magic Brush, but I haven't seen Cosmic Mario with it at all. How can Cosmic Mario stay Cosmic Mario without holding it?
-Shadow Mario does USUALLY have his brush, but he can be seen without it, i.e. when he intercepts and steals F.L.U.D.D when Mario travels through the void:
If Shadow Mario can do this without the brush I assume Cosmic Mario can as well.
3. I thought Bowser Jr's bandana he wears around his neck makes him shape shift into Mario, not the Magic Brush.
-That bandana appeared in many games. It originally had a mustache drawn on it, but it was later changed to a scary mouth. However, out of the many games Bowser Junior appeared in, he only ever transformed into Shadow Mario in the one where he had the brush.
-He did mention that a strange old man in a white coat "Gave" him the brush (Referring to E. Gadd), and it would make a little more sense for him to do that if he thought Jr. Was Mario. But I would assume Junior either stole it from him, or he used the fake mustache bandana to fool E. Gadd (he's old, he probably gets confused easily), and perfected his Mario disguise once he got the brush.
Aaaand, that's about it! I'd say it's a pretty sound theory. Oh, and if you're wondering who cosmic Luigi would be then, I'll just assume Piantissimo shared his powers with his old friend from track class, this guy: