It’s been said before but it’s worth repeating: Involve and engage your users throughout this project.
There’s no way you can begin to understand the practical concerns of everyday life for every single department in the company, nor are you expected to. The intranet should serve the entire community, so involve them from the very beginning.
An intranet team comprised of power users from every department will yield a robust system that works for all departments, not just a few.
Or, put another way, ask yourself, “If the HR department designed the intranet for the entire company, will it match how IT works and thinks?”