I've just recently hit level 30 and completed all main missions that I'm aware of, minus two strongholds. There's no indication that I have to have completed the strongholds as well (although they're my next step)
According to sources online such as this one, you should receive your specialization at level 30 and get to pick one of the three.
However, I've been level 30 and received multiple caches for continuing to level up, yet no indication of where my specialization is.
Where/How do I choose a specialization?