While Kalista certainly(T, heh) doesn't seem weak (at all, by any stretch of the imagination), I don't think her kit is as overloaded as Yasuo (the guy gets double crit chance. Why). I'm inclined to agree with most people here though; her E or passive will probably take some sort of hit, maybe a range reduction on her passive, or some sort of limit on E (either a hard limit or some sort of damage fall off, sort of like how Azir's soldiers do less damage if more than one are attacking the same target). I get the feeling that nothing is going to be outright removed, but instead a large portion of her damage is going to be nerfed and she's going to be really dependent on her support, if she actually proves to be toxic OP
EDIT: Surrender@20 just put out some more info on her as she is on PBE currently. I might have to retract some of my statements. Good lord this champion is stacked. Her passive dash actually scales with her boot rank. Not to mention she can't cancel her auto attacks (both a blessing and a curse, I guess, but probably more of a blessing, given that she can reposition herself so much better than any other ADC). Her Pierce has this mechanic where you can stack up Rend on one target and Q it for the kill, which then carries all the stacks onto the next enemy in Pierce's path. Her W passive has a cooldown and both attacks need to be within a certain time, but does %max health damage, quite a bit, it seems too (12% at lvl 1, not sure if it scales with ranks). Given her mobility and Q poke, I can't imagine it's hard to follow up on support harass. Her E does still stack indefinitely, and doesn't appear to disappear if you switch targets (as far as I can tell). At least the stacks disappear after a few seconds and extra stacks only deal 20% damage, so there is sort of a limit built into it, based on your attack speed, and each additional spear does reduced damage, her ult also doesn't appear to deal damage, and base damages (possibly ratios too, not sure, Q seems ~1.0AD, E seems ~0.7AD?) on her spells seem on the lower end, so I guess those are some things.