This can be your guiding light for planning the middle parts of your game. A lot of the fun of travelling through the story is about the player character facing challenges that they can either handle, or which are hard to them, and the various obstacles getting in their way. If there are multiple goals that the PC can strive for, and succeed or fail at achieving by the end, what are the steps of those goals and how can the PC interact with those steps in different ways?

Progress towards the goals or the finish line can be described with stats, though it doesn't have to be; the best kind of progression is also done through events the PC can engage with and choices they need to make. Those events may escalate or shift as the game progresses, building tension (whether that tension is about mystery, action, romance, or anything else) until the arrival of the climactic moments and the endings afterward.

Something else to think about is: if you have a sense of what the player character can want, it's fun to test that desire.

So, say your selkie PC wants to find their skin which was stolen but they don't know where it is. They've stated through their choices that they'll do anything, including committing violence, to get it! Then it might turn out that the thief stole it for some other purpose and was being coerced themselves. Or the thief is actually a friend the PC has become close to earlier in the game. It's become more complicated now... Does the PC double down on what they said they wanted, or do they shift their perspective and try something else?

Thinking along these lines may help you figure out what you want to put the PC through during these middle parts of the game.

Other things may inspire you too. For instance, perhaps you want your PC to have dedicated time to get to know some companion characters - so the selkie has the chance to either enter a swimming contest or join a book club, which other characters are involved with. Both activities can help the PC get closer with the NPCs, and ideally they'll develop some of the other goals too.

There's also considering what other characters are doing during the game. If it's a game with a clear villain, what scheming is the villain up to and how will the PC have to deal with the fallout of that - whether they know who the villain is or not? Even if there's not a villain, other characters will be acting in ways that affect the PC's life and move themselves and the PC towards the end points. It's a case of slotting the NPCs' and the PC's actions together so that the PC can engage with them and affect the world.

I hope this is useful, dear friend. Thank you for your kind message!