Of course its not set in concrete she will take the helm yet, its just up in the air at this moment, but a lot seems to point towards her coming to become the forsaken ruler.
What actually interests me about this, isn't Calia herself to much, but the possibility of what she could do for and with the forsaken.
Something thats bothered me for a long time with the forsaken, is that since they were introduced into warcraft, as a race they have always been very one note. Everything about the forsaken was tied directly to sylvanas and her machinations. Every twisted thing the forsaken did, came from her and her designs. And the forsaken have just blindly followed along.
It seems like many other races in warcraft have had a chance to develop into different entities. Orcs, have different archetypes from the warmongering savage to the wise and tentative shaman types or the ones who uphold honor first and foremost, they are complex and it allows someone to play orcs from any kind of angle if they so choose. Night elves you could be a nature loving hippie, or a vengeful warrior. Trolls could be ones looking to let go of their more primitive ways, or ones who embrace darker elements of their society. And so on.
But forsaken, they have been the same archetype for decades now, without an outlet for something different. There have only been a few individuals in the forsaken who stood out as being anything besides the edgy sullen emo types the majority of forsaken have been, and that's all because blizzard did nothing to develop them out of the same funk they've been stuck in for years.
So, by having Calia, and not having sylvanas around, I hope we actually get to see another kind of forsaken, one that isn't just another 'DURR ME BLIGHT THE LAND AND HATE ALL LIFE, DURR' that we've gotten for years, and hopefully see another kind of forsaken archetype for once.
Of course that same boring sylvanas archetype will remain for those wanting it, but yeah, this race as badly needed something to change it, because it just has grown stagnant at this point.
Edit: And just to throw in, those who hate this because their afraid of the change a new forsaken ruler will bring, heres a dash of irony.