the thing is that she might be pretty one-dimensional on her own,but her past can actually add more dimension to her.
It is a bit like sion who is pretty much just a vile beast for most of the time,but it is his past(and the moments where he can remember his old self)what makes him multidimensional.
It's true, her past would add complexity and the problem which has been highlighted is just this: we didn't get to know anything of her past. The short story is not about it. We're hinted something, we're showed nothing. Besides, a character whose only interesting part is in the past will eventually become one-dimensional as soon as that past is revealed. So it's a double-edged weapon. Does Kalista really have any development possibilities? We don't know, because she's never been shown from her POV, we don't really know how much her personality is lost and fogged. As of now we've got a 1-D character: revenge, revenge, revenge... That's why it'd be good if they had shown her feelings instead of telling them through the eyes of the sword-wife. We'd just have thought: wow, she's a character then, not a revenge-machine.
With how they portrayed her, she looks more like a puppet. You call her, she does your bidding (in exchange for your soul, ok, how spooky, wow, never heard of it). But does Kalista do anything to solve her grudge? Does she have a free will or is she just a cursed bundle of souls bound to wander through the world heeding the call of the betrayed? This points should have been addressed, rather than having us know about some unknown sword-wife and how to invoke Kalista. That's detailed, but... irrelevant. You could've had to stumble your pinkie against a cabinet thrice, that wouldn't really change the character, whose supposed to be the centre of the narration. After all, I'm not making up things according to my own taste, it was them saying champions should be the one getting all the attention, =/
Also it does bring a kind of diversity among the champion roster,a champ whose personality is fading away and who is becoming more like some kind of avatar(of vengance) is a unique character perspective,not necessarily a really multidimensional.
Sion's personality too is fading away and he is becoming an undead avatar of war and wrath and bloodlust. So, it's not really that different. He's less spooky metropolitan supernatural ghost and more zombie juggernaut, but the character's structure it's the same. Cool past, bad end, life after death with a broken mind and only small windows of clarity and thus them being on the brink of losing themselves completely, but with the hope of a final redemption.
Riot said that they are nowadays ok with experimenting with champs a bit more than they were in the past and that not every new champ has to appeal to the masses,thats why they changed their mind about monster champs and thats most likely why they made kalista the way she is,she is different because they didn't want yet another "muh someone was evil to me,so I want vengance and fight evil",we already have that,what would be the point in adding more?
Of course champion diversity should be a goal, but the point is: does Kalista really achieve that? Furthermore, they stated they wanted to make characters and stories which breathed and lived (<insert joke about Kalista being undead and so fulfilling neither of those>), but does Kalista strike you as such a character from the short story they published? She could as well be, if they only took the time to actually show her as a character to us, instead of having her be the Runeterran Bloody Mary.