Nothing at all, so the legendary last hidden easter egg I was looking for wasn’t there, sadly. The only thing funny to notice is that pretty much all tiles use ice physics, including the nougat platforms which are usually normal.
Though, it’s definitely pretty fun to run in these parts, and it got its purpose if someone misses any special item like a cage and they struggle too much with the pan’s controls.
By the way, Ryemanni has just shared a picture showing the first iteration of Playtopia, which he made in Rayman Designer in 2009.
He has also shown some toy world concepts he had drawn as a kid:
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EDIT: Using the UndertaleModTool, I looked inside the game’s data.win in most of the game’s versions and I found some interesting things.
-The Gameplay Demo already contained a few things from worlds beyond Band Land, like some of the sounds or the loading times screens. The Dark, Frankenstein’s monster and Jack-o-lantern skins’ icons were also in the sprites.
-The Demo also contained unused content like the original bigger seed bag, older icons for the Magician’s bonuses, and what appears to be the older SNES skin’s icon (which only changed Rayman’s colours). There were also checkpoint skins (it’s interesting to note that the Junior one was different).
-The test map Ryemanni had made to test the game’s features is still in the game, even to this day. However, it is unknown if it’s possible to get into it through legitimate means.
-The removed Woodcutter boss’s sprite was not cut (
) before the 1.1.1 update. Its sound effects are still available.
-Some other unused entities like the bat enemy or the venom originally used by the Spider Master and The Menace are still in the game’s files.
-The Ancel easter egg was added to the 1.1.0 update.
-The Globox easter egg was added to the 1.1.3 update.
-Some entities have very different names in the files, like The Menace which is referred as "pest".
(look at the top-left and the bottom-right parts of the picture)
And most importantly, I’ve noticed two very interesting things that were added in version 1.1.1 and which may be the legendary unfound easter egg Ryemanni talked about... Not only a new room was added to the game (titled "Ending3"), but also a new music (titled "Stinger"). When playing this music, it turns out to be an extract of some Rayman 2 music (the end of "Prologue ~A Ray of Hope~"), which almost sounds like a trailer of some sort... Could this easter egg be a reference to Rayman 2 or an extremely elaborate and secretive way to tease a Rayman 2 related project? The room itself is very small and appears to be black and empty, but it contains a trigger to activate some text or dialogs.
While I tried various things (mostly in the Final level as it’s the one that leads to the two original ending "rooms" used for cutscenes) and I looked into the map of every room, I found no way to fulfil whatever "conditions" are required to reach any of these things (while it was possible to see the trigger that teleports to the Ancel easter egg, so it's most likely not the same method). Perhaps this is related to Demise mode, but I don’t have the time to do it right now and cheats aren’t allowed in this difficulty... Or maybe this is related to a cheat code no one has found until now.
So, I’m sharing this with the hope that it’ll help someone to figure out how to trigger this easter egg. Now, we have at least some clues and the confirmation that there are still some things hidden in the game... It’s only up to us to find them now. 
UPDATE: I can now confirm that there are more cheat codes left to be found, as one for ReDesigner was secretly added in one of the final updates.
cages
Congratulations, your perseverance paid off!