I would love to see a close integration between Sibelius and Pro Tools where you can create a score in Sibelius while simultaneously being able, through a special linking system with Pro Tools, tweak the midi notes and the audio playback in Pro Tools (including use of plugins, freedom to choose virtual instruments that play back through Pro Tools, automation, bussing etc).
To give an example, you could create a written score for say the 1st violin section in Sibelius, entering all dynamics and technique and expression text as appropriate, and those would then be interpreted and translated into midi through the Sibelius engine, and then sent to one or more tracks in Pro Tools where you could tweak the midi note lengths, velocity, expression, exactly which VST plays back what, add automation, maybe insert plugins like EQ or compression or whatever and manipulate and automate those in real time with the playback of the score. And you would be able to set up a sophisticated routing in Pro Tools where eg the strings could be grouped, and brass, etc, possibly with additional processing per groups, and so on.
At this moment, the creation of written scores for performance by live musicians and the creation of "broadcast quality" soundtracks are still two different worlds. The standard sounds that come with Sibelius have greatly improved, and it is possible to use third party VSTi such as Eastwest, VSL, etc, but the features available in Sibelius are too limited to really tweak the midi notes to satisfactory end results.
In practice, this means for those Sibelius users for whom playback quality is essential (film composers, composers for visual media) they will either (i) start a score in Sibelius and then export midi to a sequencing program or (ii) vice versa (and probably more common) write a score in midi and then import it into Sibelius if there is a budget for parts are to be performed and recorded live.
This usually means doing everything twice, which is not just time consuming (and in film scoring deadlines can be quite short) but also expensive; and in reality a lot of film scores will be created directly in a sequencing program, with Sibelius only coming into the picture when there is budget for live players and professional orchestrators etc.
That is unfortunate, and it would benefit the quality of many film scores and increase the musical standards if composers could fully and seamlessly utilize a notation program like Sibelius without barriers or extra steps between notation and playback. Composers wouldn't be limited by their (lack of) midi keyboard skills. It would also be much easier to create a written score for the scoring stage if there is a somewhat decent Sibelius score to start with and, again, save time.
Many sequencing programs (like Cubase, Logic, and Pro Tools) have a notation editor but those are a far cry from the sophisticated and easy to standard of Sibelius. The built in notation feature of Pro Tools (since PT 8), based on Sibelius, is nice but in reality doesn't really help as PT cannot import Sibelius files.
This would require a sophisticated integration between Sibelius and Pro Tools which goes well beyond the Rewire application. In terms of pricing I would consider paying for an external application that would serve to integrate the program, if it turned out to be that the development costs of this application would make the standard Sibelius program significantly more expensive. I realize that an application like this would have little value for people who are primarily interested in the notation features.