I usually manage to clarify things before writing them down, but if I don't then
I might write it on a single sheet of paper and put it in my inbox, perhaps as
just a few words with a question mark. My "powers of 2" system can be used
in order to come across it again at varying intervals. Each time I come across
it, I can try to re-think it and see if I can write something more concrete, more
Occasionally I'll plan an action such as "Go for a walk for the purpose of
thinking about X".
There are also written techniques, such as mindmaps, or lists of pros and cons,
etc. that might help with this sort of thing. You might list as an action
"draw a mindmap around idea X".
If you haven't yet figured out how to phrase them as actions or projects,
you might phrase them as questions, and then start thinking about how
you might go about answering them.
Inability is an abstract thing involving comparison with alternate universes; it cannot be experienced.