For my Longhunter I bought a cargo chair.
For my KU 3700 I bought 2 long Pockets, a grabit and just recently a water bottle pouch.
For my Spikecamp I just have 2 Gossamer Gear medium belt pockets that I use on all my packs.
If ordering a pack like a T2 or ku all I needed was a lash kit and aux gun bearer, never needed any pockets or anything but the standard lid. The duplex is another story though. That thing is like a Lego and you can build it so many different ways. For my duplex I have a DT1 (ordered), back pouch, cargo panel, lock n load #5 and eberlestock scabbard. I had a grab-it but sold it with the T2, I never used it and feel I could do anything with the lash kit. Also the 5 string compression sac is awesome
Oh jeez, had to get a dt3 to compliment the highcamp 4800, meat bag, 1 med belt pouch, 2 large, 2 small pullouts(regret this, I have one now and kinda small) 1 med pullout, 1 mega pullout, expanded meat shelf, and a long hunter lid.
Grab-it is a MUST have with any pack. Pullouts are essential for organization. I just got 2 of the new Mega pullouts which will be great for clothing. Kifaru meat bags are super nice for many things. They are long and narrow so they fit and carry great in a packbag