'DISCLAIMER:' THIS IS JUST A THEORY, PLEASE TAKE ALL OF THIS WITH A GRAIN OF SALT
So, pretty much almost everything on World 9 is a huge mystery outside of three levels (Rescue Yoshi, Koindozer's Forest, Maze Cave 2).
Here's what I think World 9 will be like.
Black Switch Palace
Aka Rainstorm Lake.
I'm not sure about this one. Due to the name, I assume Carol is scrapping Rainstorm Lake.
It's possibly some sort of ruined Switch Palace. I'm guessing it's a puzzle level, probably like the other Switch Palaces on steroids or something.
The music would probably be Leonid's Castle.
Anyways, there are three main stages to World 9: キノコおうこくステージ1, キノコおうこくステージ2, キノコおうこくステージ3.
Mushroom Kingdom Stages 1-3.
Mushroom Kingdom itself
These setting and graphics in these might be similar to Koindozer Forest's (brown crystal representing the dried-up soil referenced in the intro, raining, pretty depressing atmosphere). The last one might be based on the Toad Town from Paper Mario and Super Mario Galaxy, since it's right across Peach's Castle.
Mushroom Kingdom 3, due to its close proximity to Peach's Castle, will probably use Hometown Domina. It may also be based on Toad Town from Paper Mario and such, but I'm not too sure about that.
These will probably be average levels. A few ASM gimmicks, of course, but mostly average.
Then, the castle level.
Peach's Castle (co-ownership with Luigi?)
Called ピーチキャッスル (Peach Castle) in the later demos. Has a neon Luigi sign for some reason, however.
I'm assuming it'll be a rather large castle level. Not as large as Bowser's, though. Music used here will probably be the NSMB Castle music. Due to the Castlevania: Dracula X graphics (the indoor segments of Stage 7) in the room with the Peach boss (1EF), I assume the rest of the stage will probably use those graphics.
The first boss will be Peach, fighting with Poochy (since his sprites can be found alongside hers). The music according to earlier demos, would be Decisive Battle, whether this will be changed in the final game or not, I am not sure. Afterwards Peach will turn into her final form, an edit of the Goddess from FFVI.
Here's how I think the battle will play out. Since Carol has a tendency to reference JRPGs a lot, mainly Square's, I think the Peach battle may be an FF parody. Mario is fighting his love interest, who is being aided in battle by a dog. She can use her powers to turn into some sort of magical being.
Rinoa is a character from FFVIII. One of the black sheeps of the FF series, alongside FFII and the FFXIII travest- I mean, trilogy. She is the main character's love interest, and part of the (rather poorly-written) story revolves around the relationship involving her and the main character, Squall Leonhart (if you've played a Kingdom Hearts game, you may have seen him under the pseudonym Leon). Late into the game she gains sorceress powers, a side-effect of being posessed by Ultimecia (the villainess of the game), and one of her Limit Breaks (Angel Wing) allows her to become a sorceress, putting her in a sort of berserker mage state (she begins spamming magic everytime her turn is up, without any player input, she doesn't lose any magic she has stocked though). Her other Limit Breaks (Combine) involve her pet dog, Angelo (Angelo is a girl dog, despite the name). These are learned through in-game magazines called "Pet Pals".
Some are activated randomly in battle, like Angelo Rush (kinda like Interceptor's Bite attack from FFVI), Angelo Search (Angelo looks for a helpful item in battle), Angelo Recover (heals a party member) and Angelo Reverse (a mispelling of Angelo Rebirth, pretty much a free Phoenix Down).
There are four activated through Limit Breaks. Angelo Cannon (Rinoa shoots her at the enemies with the disc launcher on her arm), Angelo Strike (Angelo jumps high into the air and lands on the enemy, dealing high amounts of damage), Invincible Moon (Angelo howls at the moon, making the party invincible for a certain amount of time), and lastly, Wishing Star (Rinoa hops on Angelo's back and repeatedly rams the enemies).
I think the first part of the battle will have a reference to the Combine limit breaks. As for the second part, I assume Peach will probably use her equivalent of Angel Wing and turn into Goddess (Sorceress?) Peach.
There are three sorceresses in the game, besides Rinoa. Let's focus on the one that matters most.
Ultimecia, the main villainess of the game, a sorceress from the future. She wants to perform time kompression, and kreate a world where only she can live in. She is doing this bekause she doesn't want to be killed by a legendary SeeD or something. FF8's plot is pretty bad. In the English versions of the game, she also replaces all hard Cs with Ks. She has four forms (three technically). Her first form, where she is fought normally. A battle with Griever (a leonine summon that she draws from Squall). Her second form, where she junctions herself onto Griever. And her last form, where she becomes an eldritch horror.
The second battle may use some of Ultimecia's attacks. I assume this due to Carol porting Maybe I'm a Lion, the song that plays when you battle her second form (Ulty combined with Griever). If this is true, guess we know where that song will play.
The Final Stage...?
The final stage.
さいごのたたかい (The Final Battle). Renamed きちくおうマリオ (Brutal King Mario) in later demos.
The one thing contained in this level is a teleporter room. The one from Mega Man 3 to be specific. VIP and Wall Mix 3 also had one at the final part of Bowser's Castle (Carol also worked on VIP3).
Here, you battled the 7 Koopalings plus the Zonu (an old 2ch meme, replaces the Reznors in the VIP Mario games). After this, a teleporter would appear, leading to a battle with Gamma (with graphics from the Mega Drive version of Mega Man 3 (I'm from Europe so I say Mega Drive instead of Genesis, sorry)) and Daddy Cool (called "VIPPER" on this wiki's Boss page). Defeating Gamma and Daddy Cool would result in Daddy Cool begging for your forgiveness. A weird ghost version of Newsoku de Yaruo (another 2ch meme like Zonu and Daddy Cool, also replaces the Chargin' Chucks in later VIPs) would spawn from within him, and turn into some odd mix of Yaruo and 02 from Kirby 64.
Since VIP2 and 4's bosses were in Brutal Mario at one point for beta testing, and later removed, you could assume that this one would too.
Neo VIPdeath (VIP2 endboss) and the Detteiu Chimera (VIP4 Yoshi boss) were removed. The teleporter room, however, was never removed. Even in demoOnly1, it's there, in level 135.
Also, Bowser's Castle (Daddy Cool's Castle would make more sense as a title) in VIP3 also had a Banana Bird Crystal segment, which was later repurposed in Brutal Mario in Ludwig's Castle.
My idea for this stage is that it will be a refight of all Koopalings + Bowser, a la Mega Man. It'll use music #99 as the background music for the teleport room and the fights. After defeating them all, a battle with Luigi will ensue.
Now as for how Luigi will be fought... VIP4 had a more basic Seven Heroes-esque boss involving various ASCII art characters/memes, which I think was made by Carol. Since VIP4 is from like 2007, it can be assumed that the Seven Koopalings boss is an improved version of it.
Maybe it'll be the same for the Luigi battle?
You'd fight a more advanced version of the Gamma boss, being controlled by Luigi. The song playing during the fight would be SFA2 Akuma.
As for the 02 battle... I think that'll be for later.
You'll recall that in one of the PM exchanges with Roberto Zampari, Carol commented that to unlock the final level, you'll need all the DK Coins.
I don't think this stage is the one that gets unlocked by getting all DK Coins.
I think it will be...
Sky Fortress Luigi Omen
The Black Omen stage. My idea is that after the battle, Luigi would trick Mario and escape to the Omen. Should you get all the DK Coins, Yoshi would show up (no doubt pissed off at Luigi and Peach's henchmen bombing his house), giving you a lift to the stage. Should you forget to get them all, you'd get a bad ending.
I think that there would be no paths to the stage, unlike what the sample demo implied (the one with Shinryuu and Omega), requiring you to fly with Yoshi. Flying with him would change the OW music to Prophecy for as long as he is in the air.
The stage would open with the rail shooter segment, Mario rides on Yoshi trying to destroy the Bullet Bills with Meta Knight's Theme playing in the background.
After this, Mario leaves Yoshi behind and the actual stage would begin. The song playing would be The Savior of Those on Earth.
There would be five bosses in the stage.
Halfway through the stage, a fake goal orb (most likely left there by Luigi) would appear. Said goal orb has a monster within it. You know what this is. Shinryuu and Omega. Yoshi would show up just in the nick of time to swallow Omega before he got rid of Mario.
The stage would continue, and you would reach the helm of the Omen.
I'm not too sure about this, but I think Luigi would transform into the weird mask that Queen Zeal ends up turning into (though the sprites will probably be altered to look more like Luigi). If so, his battle would be similar to her Chrono Trigger incarnation, with Battle With Magus playing.
Maybe the Queen Zeal sprites are only being used as a placeholder until his sprites are finished? Peach and Poochy's sprites were taken from other games (Super Princess Peach and Yoshi's Island), with the second phase being merely a recolor. Turning Zeal into Luigi would probably take a lot more work.
Afterwards, the true final boss would appear.
Thinking back to the 0² battle in VIP3...
The battle to restore peace to both the Mushroom and Koopa Kingdoms would finally begin. Mario would fly towards the sky and battle Zero, much like Kirby did. The song would be Missgestalt.
After this, Zero will transform into0².
The song changes to Rising Sun and Mario then proceeds to finish him off.
If you'll remember, in Kirby's Dream Land 2 and 3, and Kirby 64, the main plot of those games revolved around Dark Matter posessing the denizens of Dream Land (and in Kirby 64's case, also taking over an entire planet) and turning them evil.
My idea is that Mario got infected by Dark Matter when he was crowned king, and the denizens of the Mushroom Kingdom also did as well when they swore revenge against him. Zero has a tendency to try and make everything horrible for no reason at all (guy has no backstory), so if it's really him, it wouldn't be too surprising.
Also, in level 1F1 (the one right after the Black Omen's helm) Mario's sprite is recolored white and red. His entire palette (click the star between the castle and the blue block with the magnifying glass in the upper bar) is changed to white, red and black. Zero's colors.
There is nothing in that level. No layout, no custom graphics, no nothing. Yet Carol went out of his way to change Mario's starting position and also change his entire palette? That's... Kinda suspicious.
Cue Credits Concerto.
After this, a celebration at Peach's Castle would be made, congratulating Mario on defeating Zero. Apologies would be made, friendships would be restored, everything would be back to normal. Mario would then be re-crowned king. Following series tradition, everyone would go to eat some of Peach's cake after this was over.
The credits would follow. To Far Away Times would start playing.
'ONCE MORE:' THIS IS JUST A THEORY, PLEASE TAKE ALL OF THIS WITH A GRAIN OF SALT
THANK YOU ^^