You can put a water bladder in any pack. Of course, many of us have ditched the water bladder and gone back to the older technology of bottles.
I would say that if you already have a pack, then look at the rest of your gear first before changing the pack. First look at your sleeping bag and sleeping mat. Then move on to shelter and clothing. After that (and several trips) you will know what kind of gear you need. Then a pack is easier to either buy or make, because you know what you need to carry in it.
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