I have a directory. Every idea is either on a file, or a subdir that holds whatever work I've done on it. That subdir may hold a TODO list, scoped on that idea - if I ever work on that idea again, I'll probably start by reviewing the TODO.
I don't intend to work on 90% of what's there anymore, it's not a TODO list. It's just a set of info that I may use someday later. I don't even know what's in there.
Good point. This is also what I do with projects that have some (partial) implementation. Local TODO/IDEAS files are a very good thing.
However, most ideas didn't come to live (and probably never will), and I prefer to have those in a single file instead creating a separate file or directory for each topic. The reason is that some ideas are vague, and sometime I split or merge them.
I don't intend to work on 90% of what's there anymore, it's not a TODO list. It's just a set of info that I may use someday later. I don't even know what's in there.