Let's get this ball rolling...
Well, I have four ideas off the top of my head:
1. Improved MIDI capabilities - for example, control over note velocities (recording and playback) and various randomizations. (I know Sibelius's focus is on notation, but if more MIDI features were included I wouldn't have to use something like Finale or Sonar for recording/performing my Gigastudio stuff, and I find Sibelius a BILLION times easier to use 
). Maybe even a graphical interface mode similar to ProTools' MIDI 'blue bars' 
where one can rhythmically reposition notes and chords ever so slightly without the rigmarole of tying and untying hemidemisemiquavers (or 64th notes 
) ? 2. More fully automated layout - okay, it's improving noticeably in every version, but the two main standout omissions for me still are a) slurs, phrase marks etc. and b) lyrics.Big phrase marks can require quite a lot of manual work to avoid hitting everything; it would be nice to have a kind of collision detection system to calculate the optimum arc for for a slur, not taking up too much space, but not crashing into every stem in its path.
The lyrics also tend to ride up into anything remotely poking down below the staff, especially low-register notes and articulation, and our old friends those slurs and phrase marks. It would be lovely to have the words automatically position themselves at a level low enough so that they would avoid all these sorts of collisions - it is very time-consuming to arrange and re-arrange these things when the music is always being reformatted, as is my wont.
As part of these two suggestions, getting Sibelius to move the staves around automatically (e.g. spread out or contract) would help to free up the necessary 'physical' page space for some of these processes (perhaps implemented with a toggle in preferences or only affecting unlocked passages).
3. The ability to create bracketed cautionary accidentals automatically - there is a plugin by Danny Manners that puts accidentals in (with highlighting and/or text for good measure), but a bracketed version would be nice - perhaps just having the facility to put brackets around things more easily would be a start. 4. Having a broader range of options in the playback dictionary. I love the idea of the dictionary, and I guess two main changes would be beneficial for me: a) supporting additional facets of playback and b) being able to pass parameters.The additions would be things like various 'lines' simulations and partial overrides (you'll see what I mean soon). Octave displacement, in fact transposition of any size, would be a nice feature. In the Garritan Orchestral Strings sample library, among others, there are sets of instrument sounds with up- and down-bow samples, separated by, in this case, three octaves - if I had a dictionary option for instant transposition and could override the upbow marking, I could tell Sibelius to automatically play everything I mark upbow three octaves up. Even if I couldn't override the upbow marking, I could write a script to put in a technique line saying "UPBOW" everywhere there's a ^ (some imagination required here), but the overriding would be neater, in a way. In any case, I'd enjoy the freedom of more options there.
The option of parameters would be useful for some MIDI messages, where you want to send similar messages but with different associated numbers.
Sibelius is already outstanding, but with additions such as these, I think it would keep on improving in usefulness and productivity.
I'm looking forward to hearing everyone else's suggestions.
Julio
