Without changing gameplay, it might be helpful to make the "Choose a challenge" button read "Choose a challenge, subplot, or goal". Bear with me. This implies that you are choosing what the objective of your character is in their Move.

Then, if you choose to pursue a subplot or goal, you are prompted to choose a challenge that is pushed towards "uncertainty".

In essence, it's still the same as the system is, now... it's just clearer to players what it means to play a goal or subplot.

From what I can gather, the idea of a goal / subplot card being played on a challenge and leading it toward the unknown... is that your character has decided to pursue a personal task, letting the scene challenge fall by the wayside.

Sometimes, players aren't using these cards to actually pursue these personal tasks in their writing - only to fill a slot on a challenge.