be careful - there are so many note taking apps out there, and it's easy to go insane making one.
i've been warned myself by members here about do i really want to embark on such a project (regarding clipboard helps+spell and making a new version of it..)
see the letter by the author of keynote on him burning out from working on the never finished keynote2 idea in another thread as well..
but it's very tempting i must admit. for a programmer it's a fun project.
the best advice given to me if you are determined to work on it is to check out keynote2 discussions for ideas. though i guess my first advice would be, are you sure there isn't another project you might want to work on that doesn't have so much good competition..