(Specifically, I would rename "subplot" to "motivation". Then playing it on a challenge becomes "this is *why* I'm acting this way" and it works. "Subplot" and especially "Goal" imply that it's something you have to work towards separately though, which really doesn't work with the current mechanics.)

My thoughts exactly.
Although I do tend to find that, most of the time, Subplots make enough sense. I probably would rename them, but if you model them off the ones in the standard stories playing them on challenges works out pretty decently.

I can also see how narrators would have occasional uses for the current Goal cards. I just don't feel that that use is for giving characters goals.

...Something that I only now thought of is the options to play personal challenges out - like you can do with goal cards currently - without giving them to an individual player. I guess that should be an option? I mean, I don't see why not.

I love the idea of Personal Challenges that carry over scenes. Whether or not Goals / Subplots are replaced... I have a feeling.. is not likely.

However, I know I would just personally end up using the Personal Challenges instead of goals/subplots. They're definitely needed in the story I'm playing with my group, and would work far better than goals / subplots.

Sometimes, I see players end up playing goals / subplots nonsensically on challenges. Plus, they can't be played unless there's a scene challenge, which is an abstract and bizarre concept in and of itself. Why do I need a scene challenge to exist for me to play out a personal plot line?

I suppose just adding player-specific challenges that carry between scenes would be alright as well, but I feel like that would just have too much overlap, use-wise, with Goals.

See also https://storium.uservoice.com/forums/211322-general/suggestions/5822515-implement-multi-scene-challenges , which is somewhat related.

I'm aware there are some issues related to this idea, so I'm going to go over a couple and my proposed solutions.

Firstly of course this is a major change. I don't feel that's an inherently bad thing, but it could be a bit jarring for some narrators. Presumably existing Goal cards would remain as they are, and existing games would retain the option to use the old version of Goals.

Second is the question of outcomes: since they would work more or less like challenges, presumably the narrator would define outcomes when they create the goal. However, these outcomes could become obsoleted as the story continues on, so there should probably be some mechanism to edit them, even if it isn't used often. OTOH if the situation is rare enough it might be fine just to leave it to narrators to handle it in the comments when it does occur.

Thirdly is the question of rewards; players may not have enough incentive to play cards on Goals if their only reward is a single wild strength. I personally think that would be fine (since playing out strengths and weaknesses is also good in and of itself) but perhaps we could just give the wild strength as a basic reward, and allow the narrator to add additional rewards on top of that?