I would like to see the scoring functionality in Cubase expanded to have similar functionality (if even just a small subset) to what you would find in software like Guitar Pro. Having that functionality integrated with Cubase would make it so you can compose with the actual instrument and effects intended for the composition. Expression maps and re-assignable CCs would likely make setting this up with third party virtual instruments fairly painless, with added note expression functionality being available for VST3 virtual instruments. Guitars and Basses in Halion Sonic could be set up to work with the new scoreable articulations right out of the box.
In particular I am looking for articulations like bends, the various slides, tap, slap, pop, strum direction, etc.
I don't know how closely Steinberg works with third-party developers, but after getting it worked out for Halion Sonic you might consider making sure it is fairly easy to set up with virtual instruments from a handful of the most popular third parties like MusicLab, Spectrasonics, Vir2, Native Instruments and Ample Sound.
I know Steinberg is working on separate scoring software right now and hope it will include a good range of guitar articulations as well, though in particular I would like to see them available in Cubase where they actually need to function with virtual instruments.
Along with VST Amp Rack I think functional guitar articulations would make Cubase the no-brainer go-to DAW for guitarists as well as for composers in general that tend to feature guitar in their tracks.