i had a very formulaic means of design in smb when it came to the level design, and how levels introduced new elements and challenged the player in new ways as the game progressed. i knew from the start on paper that i could make it so the game didnt get old at all a while before even making the levels or mapping them out at all.
if you pick apart each level of smb beyond the 1st few, you will notice a pattern that goes like this.
intro new gameplay mechanic (ie keys) with a level that shows how they basically work.
next level.
reuse the same mechanic in a new way (ie using key blocks to push the player through a level).
next level.
reuse the same mechanic but comboed with a previously introduced level.
next level.
intro a new gameplay mechanic.
thats a very basic outline of the whole game on a broad level, but a more intricate look might also notice patterns like.
small level-
med level-
large level-
trying my best to not only use level size in a way that mattered to the new mechanic or gameplay in general but also to make sure each level felt fresh. i do the same thing with level colors, themes and the like visually.
going even deeper you will find precursors and throw backs.
a precursor would be a tease of a new enemy or mechanic that would be appearing later in the chapter, and a throw back would be a basic mechanic introed early in a chapter that would be revisited at the end with a much much higher difficulty level.
i also tend to design levels, chapters and the game it self with symmetry. (ie the final chapter of the game mirrors the 1st in its mechanics)
i put a great deal of thought into every aspect of that damn game, and i probably went beyond what you asked but it got me thinking and wishing i was working on another platformer :)