About the key signature (armadure), if the user choose, for instance, the key of G major (with the f-sharp) and click+drag the mouse it will get only the seven notes from the G major scale? if thats the case I really think that one important option should be the chromatic scale and in that case the user need again to choose wich notes are flat and wich are sharp.
If you are planning to make the object behave in major/minor scales the really best would be to have the options to accept any scale like, for instance, the modes of limited transposition (http://en.wikipedia.org/wiki/Modes_of_limited_transposition) and a lot of ethnic modes wich are about unlimited (and a lot of them are microtonal, by the way). If this is too much, at least the chromatic option is imperative. On the one hand, the scale behavior is nice for some calculations (I already have some ideas), but honestly I would almost always use it in the chromatic mode.

I had an idea for the mouse dragging that I don't know how hard is to implement.
How about making the object behave like the number atom, that is, if the user click and drag, the notes will go up and down chromatically (or in the scale choosen) and if the user shift+click+drag the notes would move in a microtonal way. It doesn't have to be that much precise (I would say, two float points in midi scale maybe? = cents), if the user really wants to make precise notation it would input it as a float on the inlet. At least in western music, this much precision is impossible to execute by a human player playing an instrument, this precition would be only for calculations anyway.

2010/11/9 Joćo Pais <jmmmpais@googlemail.com>
If i'm not mistaken, in PWGL and OpenMusic the user can choose if he or she
wants to display all the notes as sharps (dičse) or flats (bémol) and then
change each note to its enharmonic pair if wanted. What I supouse would be
the best choice is to have a kind of message that you send to the object
with the following options:
- all "black notes" as sharps. suggestion: [accidental sharp<
- all "black notes" as flats. suggestion: [accidental flat<
- wich note I want in wich way. suggestion: [accidental 1 0 1 0 0 < meaning
c-sharp, e-flat, f-sharp, a-flat and b-flat.
What would be extremely nice is if the user could change those options after
the note is displayed, dynamically.

In the case of microtones, again, if I'm not mistaken, in those CAC
softwares the user can choose between:
- Display the quarter-tone default symbols(
http://en.wikivisual.com/index.php/Image:Partial_accidentals.gif) if its
just quarter tone what the user wants
- Display the arrowed accidentals for "more-than-sharp",
"less-than-natural", etc. (see attached picture, made with lilypond) and
display a number at the right of the note wich I don't remember if it is the
frequency in Herz or the midi pitch as in, for instance, 60.3. I personally
prefer the second option, but I believe that in the perfect tool the user
could choose between the two.

I don't use open music, but can get the input of someone who works with it almost every day. The list Caio made is right from a notation viewpoint. The only thing that can be added is that for the microtonal pitches, the number above is usually either the frequency of the note, or the cent deviation (between 0 and 100).

Or maybe any other number that the composer wants to display there. But at some point, it is expected that a number will appear there.

Joćo