Ditto on the Eberlestock 2 liter pouches - Great for smaller items you don't want to lose in the main compartment. The G2RC rain cover if you'll be out in any and all condtions. I've also used 5.11 tactical 6x6 pouches on my GSII. The 6x6 medical pouch from 5.11 works very well - sometimes mount this inside the main compartment to the molle there. 5.11 single and double pistol mag. pouches work well for spare pistol mags., flashlights, spare leatherman, etc.
Don't waste your money on the Endo frame whatever you do. It's cheap enough to try, but it just doesn't work well. Save your money here.
The pack scabbard and it's cover do a fine job of protecting any weapon put in the scabbard.
As for the saddle for shooting - I've never seen the need. Rifle has a bipod on it, so I use that, or lay rifle across pack.
If you think you really need more, it could be that you should consider a bigger & different model pack.