I do plan on both having unlockable Specializations, as well as upgrades to the one you start with. The Scavenger specialization will probably be more useful when there are more Delves to access, but it might well be in need of a buff, too. Right now, it basically makes it impossible to find nothing when scavenging a Delve and increases the odds of getting better stuff as well. 

I don't want the starting Specializations to be easily attainable, or even attainable at all, in one play through, as it would result in players just hot-swapping the optimal setup for every situation, which would be tedious. Having those be unique encourages repeat play throughs as well. Any that are unlockable will be hard to acquire and will be different from the starting ones.