for example, when someone promises me a book, I'd like to add "pending" or something like that to the entry.
That's what I use the "granted" option for - I have no need or desire to keep a permanent list of all the wishlist books I've received (beyond my "thanks for the wishlist book!" comments in my own JEs!), so I use "granted" on promised-but-not-yet-received wishes. (I suppose having an actual "pending" option in addition to "granted" wouldn't hurt, even though I don't need it myself.)
As for editing existing wishes for spelling, etc. - that would be nice, especially if it also involved detaching the wishes from the "global overview" data. At present, any new wish creates a new global-overview page, with some unfortunate side effects. [The one that bugs me most: adding a new wish for a book/ISBN that isn't already listed results in a global-overview page that does NOT include the specified category, and this means that whoever next registers a book with that ISBN will, by default, get a book with no category (as per the "inherit the nearest global-overview info" feature/bug). This can be fixed by editing the book details, but it's really, really annoying.]
Anyway, if wishes are not uncoupled from global-overview pages, then an "edit wish" feature could be tricky; would it cause new, slightly-different global-overview pages, or would it edit the global-overview page (which might not be a good idea - one could want a wish to say "or any book by this author", while one would NOT want the global-overview page for a registered book to include such text).