I have an old Shimano bag that is perfect, but here are the things I look for: Place to hold rods and rod holders so I don't have too much in my hands - enough room to hold my reels and rigs.
This is the stuff I keep in my pack:
lures, hooks, sinkers, extra line, rigs, knife, line cutter, fishing license holder, gloves, fish lip grabber, water, scale and tape, headlamp, mosquito spray, poncho
I keep a notebook with baseball card holder to keep my various types of rigs and hooks in and a box with lures and additional knick knacks.
Think that should be a decent start, might've missed some stuff I have in mine tho