Do we know it'll be a "talent" system at all? It might be close to D3's paragon system, i.e. giving you a long-term way to progress your character incrementally.

As for swapping out ultimates, there's a lot of potential there - they could add that as a new slot one day, or they could create "subclasses" for each Javelin (similar to the specializations in D2) which are distinguished primarily by changing out their melee and ult.