I think Kalista's got potential. You've just got to do something with her other than VENGEANCE!!!!!!111, because that's such an incredibly cliche motivation.
So what else do we have going for her?
- Greek myth—Pantheon aside, ancient Greece is a largely untapped mythological source in LoL. I've always thought that the Furies were pretty badass.
- "Sea of souls"—She's driven to find other vengeful souls and bring their quests for vengeance to completion, but as she does so, she loses more of her own identity.
- Shadow Isles—She's got a hook with the Shadow Isles generally. Fits nicely with Greek myth. CrazedPorcupine recently suggested that maybe it was her own son (or other family member) who betrayed her, siding with the Ruined King; that's a richly classical setup with some good material.
- Supernatural—The fact that she's a sort of vengeance demon does give her extra material to draw from. I mean, I guess Yasuo's out for vengeance of a sort, but he's also just a dude with fancy hair and a raised collar.
Because vengeance plots are so intensely cliche, you really have to either work in some other theme that's weighty enough to hold up in those deep, tropey currents or else just do some seriously fierce investigation into the theme of vengeance and its psychological complexities. (I mean, okay, there's a third option as well: do neither and just be okay with a fairly flat character. Plenty of action movies and whatnot do that, or something similar: I MUST STOP THESE EVIL PEOPLE! THEY HAVE KIDNAPPED MY FAMILY!) What's absolutely deadly is giving a vengeance-motivated character a throwaway treatment.
I don't know if I'd agree on those other characters, though.
She's got the "rogue Demacian noble" thing going for her, too. And the "holy inquisition" element.
He's at least got the "holy inquisition" element, or something very much like it. And I guess the idea that Senna's soul is still in Thresh's lantern adds a unique dimension.
All I've got to say is…Renekton Bot wrote a great piece for him. If I fault it on anything, it's on the somewhat one-dimensional portrayal of the League, but that's mostly just me being hyper-sensitive on that issue given recent developments. The League probably does come off as douchey and manipulative in some contexts.
I think what it really comes down to is…vengeance is, perhaps appropriately, a character element that's quite tempting and sounds great on paper, but is actually pretty cumbersome and potentially corrosive. As a secondary element, it's pretty flexible, and provides a nice "character progression boost" in the short term. As a core motivation, it's really likely to launch the character straight into a brick wall of mundanity and dead-end plot.
Like, for example, Riven hates Singed…but she's not a "I must destroy Singed at all costs!" character. That's just part of her. It combines with her devotion to classical Noxian ideals to produce a much more complex character element that's a lot more flexible story-wise. If you rewrote Riven as just like a vengeance-missile aimed at Singed, I think she'd be a lot less compelling.