You’ll encounter most design problems in the end (?)

This may not apply to all, but it did for me, so here we go:

You’ve taken your time to create great gameplay and a great game-loop. The game is fun, most problems seem solved, but really there’s still lots of work lurking in the corner.

After some amount of playtime you’ll often find many loose ends for situations you haven’t accounted for yet. Different design decisions come together to produce all kinds of edge cases that need to be handled one way or another. These are the kind of tasks that can add up weeks of design and development time before you actually finish the game. Adding something small can also turn out to be a huge task, needing to support everything else that is already in the game. This applies to both design and programming. Don’t underestimate it.