Kalista "The next Varus"
To start off I am basing my opinion off of what I have seen and heard, I have not personally tried her on pbe yet (seeing as i do not have a pbe account )
From what I have seen so far Kalista is going to be very weak in a live setting. For a few reasons. First she is going to be a carry that is dependent on the support that is picked. Let me explain myself. Her ultimate "Fate's Call" takes the "Oathsworn" and throws them in a targeted direction. Now in game this item is usually given to the support because of the passive on "Sentinel". I for see a problem because of this, certain supports will have to be played with her. More particularly tanky supports such as Thresh or Braum. This is because they are thrown to the center of a fight for an engage. This issue being when people pick Kalista you will almost be forced to pick one of these supports. Any other supports Soraka, Zyra, Karma, Janna, or Zillean will be poor choices, because these supports will just die when ulted into battle. Almost if you sent a charmander out to battle a squirtle. I CHOOSE YOU KARMA!
Now to get to why I named this why i did. Varus is a carry with very strong poke potential, who suffers from a lack of mobility. His ult "Chain of Corruption" snares an enemy and surrounding enemies if they remain in the area long enough. The thing here is this ult is very comp dependent. Meaning you need the right team to make his kit useful. Im afraid Kalista will fall into this category. Why are Marksmans like Lucian and Tristana picked? Yes they are very strong carries but thats not the reason they are picked. Any carry can do damage from Autoattacks. Hell build static shiv infinity edge on LeBlanc and you will do damage to anyone. The point here is that they fit into nearly any comp. Lucian has good lane presence, a solid team fight, and great siege potential. Varus is not picked because he isnt versatile. Which is what i fear will happen with Kalista. She has an ult that is dependent on other champions. A kit that forces marksman and support to be one. A great portion of her damage comes from the combination of both champions attacking. I can not think of a situation that this would be benificial to the player. Solo que will be difficult because you have to depend so much on your support. Duo que with this champion will work but that is frowned upon in most cases. From a competitive stand point teams will have to pour too much effort into bot lane and form comps around it, leaving weaknesses in other parts of the game.
Potential ideas for fixes (feel free to add your opinion too :)
A drastic thing that could be done to fix a lot of the things discussed above. Just turn "Oathsworn" into a buff that lasts for a set amount of time. So when mid game hits and team fighting starts Kalista can give "The Black Spear" to your jungler or top laner. This way when you ult you can use your primary initiater as an ingage allowing you and your support to remain safe while your top lane, lets say, Maokai starts the fight. Now I know you probably wont even think about doing that because the passive is supposed to be like a bound by souls Marksman and Support. So as an alternative for this give the ulted person additional resistances to make their journey out of the center of the fight a lot more pleasant. Doing either of these ideas would make her more playable. And your team would not be so dependent on your pick.
Well that is my opinion on Kalista. In no way am i complaining i thank Riot for new content i just dont want another champion to not have playability. She has a cool design that integrates orb walking(which is sweet!!) And an awesome W that sends out sentinals to scout the area, which falls right into the new season. Excited to see what people have to say about this.