"When we’re doing an action game, we make the second level first. We begin making level 1 once everything else is completed."
- Nintendo's legendary designer Shigeru Miyamoto, in a candid conversation with Dragon Quest creator Yuji Horii in 1989.
The topic of conversation was about how both creators find that the level designs in their heads (or on paper) end up being a little too difficult in practice.
Miyamoto goes as far as to say that (at least at the time) his teams reduce the difficulty of his game's levels by around 20 percent once the game is complete. Horii says that the Dragon Quest dungeons he designs on paper are "outrageously difficult" in practice, and he's usually forced to adjust the designs later.
In a world where many developers find themselves forced to polish a game's opening sequence first in order to get a playable demo out, this 23-year-old advice might not seem feasible, but I think there's some golden wisdom here.
Your first level (or tutorial, or sequence, or whatever you want to call it) should serve as a prologue for the rest of your game. It should introduce many of the concepts your player will be interacting with through the rest of your game, and it should do so in a way that doesn't alienate them right away.
And what better time to introduce the rest of your game than after it's done, and you know it inside and out?