It's not possible to answer which specializations the assassin should have without knowing what the role for the assassin class is. If the assassin is supposed to be for example a kamikaze melee nuker, then it's poiintless for me to suggest specialization for being ranged or a tank or DoT. So what is the role of the assassin class? Is there any other class with the same role? Which class is better at the role then?
But I'll say this at least because I saw someone say that assasssin is supposed to kill healers in the enemy back line. But that's not realistic. First of all, it's not easy to kill someone 1v1. If assassin can quickly kill anyone 1v1 then that's OP. But for that kind of role they havee to be OP because they don't have time to start a 5 minute duel chasing someone in the enemy back line because there will be 10-30 enemies attacking the assassin very fast who is all alone in the enemy back line trying to kill a healer. So that's a fantasy role, not realistic.
I think it's better to delete assassin class and make the ranger have an assassin specialization instead. But I guess it's too late for that kind of suggestion. So I think what you need to do to make assassin viable is to think about assassin when you design the map. For example if there is a big battle to capture maybe a castle, then the assassin can sneak to secondary smaller objectives that aren't guarded by so many enemies. Assassin is not good in the main big battle because assasssin is too squishy, its a melee glass canon class typically, there are too many enemies. Assassin just isn't able to be as effective as a warrior/paladin or ranger/mage/priest etc in a large battle. So there needs to be smaller objectives they can go do which would be a waste for other more important classes to go do.