I saw one user’s suggestion for a Construction themed world in a 2D
Mario. You argued that it would be difficult to justify, in itself.
However, it could be done as an optional set of stages that unlock as
you play the game. It would take effort (something we all know
Nintendo developers are loath to do with 2D Mario), but it could be
done as follows:

When the user first comes across the area, it’s just an empty lot. It
wouldn’t be too different from the wandering Hammer Bros. or World 2
Desert from Mario 3, where there is either enemies, or the landscape
itself is a flat course with some minor obstacles (cones,
wheelbarrows, etc.). After the player completes a fortress or palace,
another level will unlock, which will be the level at a more advanced
state of construction. For example, some of the framework will be in
place, and there will be more advanced platforming elements, as well
as more difficult enemies. What once were Goombas, would be Koopa
Troopas or Buzzy Beetles, which would give way to Hammer/Boomerang
Bros., etc.  As the user completes each level, it’s closed off, and
unable to be replayed immediately. Because of this, we’d have the
illusion that the building is being built, even as the player
continues onward in the game.

The final level of the stage would see Mario running through the
finished structure, the view being seen through giant picture-windows,
jumping between furnishings and fixtures as he dashes to the roof,
where a secret boss awaits. To really tie things together, make it a
boss from a non-Mushroom Kingdom location, like Wart, Tatanga, or (if
they want to use their weaker properties) a character from their other
Mario titles like Rawk Hawk or a Wario/Waluigi duo. So, basically,
you’d be unwittingly helping Bowser’s rivals through the process.
Anyway, defeating the boss will unlock the ability to return to the
previous stages in the Construction area something, like, say, the
ability to switch from day to night at will, (to reference another
previously-mentioned idea) or something else to change the player’s
view of the world. A reclaimable Hammer Suit? A hidden battle mode
game set in the building? Something fun that the player would enjoy.

Now, if we really wanted to get involved, we could have branching
logic. What if the player doesn’t finish the levels and just goes
forward? We could have construction continue, though the building
would become more like a Koopa fortress than a standard building,
which would lead to a different boss battle, and a different reward
when the tower finishes.

The idea of locking things off is to give the illusion of growth.
After all – you can’t go back to the open pit of a construction site
after the walls are up and the furniture’s in place, right?

Anyway, this is just my two cents on how such a feature could be implemented.

All the best,

It seems a little gimmicky. And you want each level to stand alone.

But it did get me to think about Doki Doki Panic and Randy Glover’s Jumpman. There was a stage in Jumpman where when you got the ‘coin’, you got a piece of a ladder. And you could place that ladder on any other ladder to make it longer. You’re kinda ‘constructing’.

With Doki doki Panic, you would carry things such as blocks to make a platform and dig tunnels in these gigantic sandy depths. This is something that could be expanded on. Although, I don’t think an entire world could be based on it.