GPO4 has a lot of the right stuff already, so I have been thinking a lot about this question, about what, in addition to that already suggested, might make GPO5 a grand-slam in my book.
I started with GPO4, and have since added (an incomplete) VSL SE + MIRx to the palette. While I have been learning much about stereo imaging, EQ, and mixing - not being an expert at any of those skills, I have looked for solutions that allow me to focus primarily on learning to write/sequence orchestral and chamber music, without having to 'create' balanced templates first.
I would like to see GPO5 expand and improve upon the existing pan/stage depth presets, such that an entire orchestra* could be loaded within a limited choice of venues, already placed inside the stereo field, mixed/EQd, with reflections / reverb applied.
* preferably as 'divisi' as possible
Right out of the box, one would have a realistic sounding ‘personal orchestra’ ready to write for in either notation or DAW software. If desired, the option to focus could be on learning traditional composition, the nuances of orchestration, and balancing dynamics - while the software provided decent aural feedback on one’s success at the task.
Sound quality should/would need to be such that using the ‘out of the box’ presets would rival or surpass the quality of currently oft recommended 'starter orchestras', such as EastWest's options, or for notation software, NotePerformer. Ideally, GPO5 would be of a quality that such suggestions would no longer be made about 'other' orchestras.

Of course, equally as important is having access to the dry samples, status quo.
I also much prefer Garritan products copyright protection method to almost anything else I've seen. So uh, I really really want the status quo there.
I think GPO is a great product, and I look forward to seeing what you come up with for it’s next iteration Gary.