This is exactly something that I have been trying to design in my spare time recently, and I want to suggest a feature. Building off the "Postpone Tasks" ability: postponing tasks is a bit of a reverse spaced repetition, isn't it? A task comes up that you asked to be reminded of, and you can dismiss it by asking to be reminded again later. That's a lot like being shown a flashcard in spaced repetition software, and after review deciding whether you want to be shown it again quickly, in the near future, or in the distant future.
An element in spaced repetition that usually isn't treated well in software UI is how you deal with a card other than by setting a new time on it. When you're reviewing the card, you might decide that card is bad, and you need to delete it; you might decide that the card is badly worded, and you might want to correct it; or you might decide that the card is too complex, and you want to break it down into multiple, simpler cards.
Imagine that in a task manager. You just type a stray goal to yourself, and throw it into a short-term, medium-term, or long-term bucket. Maybe into a priority bucket, that starts off in the medium-term and with every dismissal gets a shorter and shorter reminder period; and a ideas bucket, that starts off the same as a priority, but backs off instead of becoming more urgent over time. Imagine easy controls to turn these stray ideas and priorities into tasks, and to further divide those tasks into subtasks.
Eventually you'd have so much stuff floating around in there that you might be able to ask the task manager what you could be doing when you have downtime, and it will start bringing up things that you are interested in, and that fit into the time you have and your location. In the morning, when you first open it up, you'd have your scheduled appointments, but you'd also have a bunch of (your own, previously deferred) suggestions for things you could add.
In that way, you can help people create tasks and goals by encouraging them to type stray thoughts and ideas in, to be reminded about them later for expansion into tasks or for scheduling.