Basically add “learn spelling for this document only” option when right-clicking a word marked as incorrect. I think this distinction would be helpful because sometimes the spell-check simply does not recognize a normal word, either because it is an obscure one or because I am writing in a little-used language for which Scrivener has a limited vocabulary.
On the other hand, I generally also want to add made-up words, such as character names, into the word list and if I don’t intend to use those same names often and some of them have similar spelling to other words, it would be better to get rid of them when working on a different project.
This has been brought up before, and we haven’t entirely discarded the idea, but it has been put on hold for technical reasons. On one platform it would likely be entirely impossible, and on the other it would not be easy to do. We rely upon third-party spell check systems, and it’s not always possible to alter the fundamental behaviour of them.