The backpack accessories you need, like most things, comes down to what your specific usage is going to be like. There's never going to be situation where you wish you had less storage space, so usually what's essential is exactly that - additional pouches of carry bags that you can attach to your main backpack. Outside of this, another essential would be a bladder with a drinking valve so that you can easily carry a supply of water that's accessible while on the move. A lot of backpacks come with space already installed for these types of bladders, so you'll just want to make sure that you pick one that fits in your bag.
There are a few accessories that you absolutely must have for your backpack if you're planning on using it in a survival situation. Water is the single most important element to survival for humans, so you'll want to make sure that you either have a bladder and drinking valve, or a suitable MOLLE attachment for a decent sized drinking canteen. Having extra MOLLE webbing and attachments is an awesome idea to add to your backpack because it means you'll never run out of space to carry your gear, and likewise it won't be dangling and falling all over the place while you're on the move. We'd also highly recommend a decent first aid kit as an accessorie to add; you never know what or who you're going to encounter and dangerous situations can arise very quickly - so having the ability to treat, overcome or at least slow down the impact of an injury until you find more help is essential. From here, it really depends on what you're going to be using the pack for that should define what your accessories are - but one thing is for sure, you can never have too many.
Military backpack accessories are most commonly used in the form of extra storage space. Whether it's a MOLLE pouch or a rifle holster, anyone who's been in a survival situation or in the military knows that you can never have too much storage space.