Changing the topic a bit, anyone seen the theories going around of Super Mario Galaxy 2 possibly being hidden within the Super Mario 3D All Stars collection? I've watched like three different videos on YouTube of people discussing it and it all originated from Nintendo Minute's video on their first impressions of the 3D All Stars collection. When the collection intro played, people caught on that the background themed used was one from Super Mario Galaxy 2. Some have argued the possibility that Nintendo slipped on the editing but, something as simple as that? Mmm, I dunno.
The theory got me thinking on the original Super Mario All-Stars game that released on the SNES (now available for free on the SNES NSO Library for NSO subscribers). Although All-Stars didn't have unlockable games, it did have Super Mario Bros. The Lost Levels. Originally, this game is the actual Super Mario Bros. 2 (was in Japan, but on the US it was considered too difficult, and those gave the number 2 to the SMB game we know today; the one with Wart as the main antagonist).
In addition to the original Super Mario All-Stars, the SNES Classic Edition also included an unlockable (although before it released, people knew what it was already) in the form of the never released Starfox 2. Who's to say that in the Super Mario 3D All Stars collection, Nintendo decided to go ham and actually include Super Mario Galaxy 2 as an unlockable (requiring beating either just Galaxy, or all three games)? For now it's only a theory that's been doing the rounds recently but, if it turns out to be true, Nintendo's got a couple of tricks up its sleeve still.