Azir's old W would let him literally sacrifice sand soldiers into the towers for damage, leaving a champion with incredible ranged-safety and tower pushing potential. This was removed and they continued to nerf him into the ground to keep his balance in competitive play. Azir also brings an actual turret with his passive, something that will CS for him and threaten an enemy team moreso than anything else on the map (aside from, perhaps, Mordekaiser's Elder Dragon). He's still a dominant force when played well, and is absolutely game changing in the right hands. Check out Season 6 Immortals vs. Team Liquid with Pobelter on Azir.
Kalista has always been in an awkward spot thanks to her passive and %Health Damage on W. It scales very well as she progresses in the game with more Attack Speed and Tier 2 boots, and she is incredibly slippery (if not slowed/CC'd) and can kite an entire team back effortlessly with Attack Move. She also can push the wave very well from level 1 while keeping this safety, and her Rend (E) stacking up to 255 stacks being able to out-Smite even Nunu and Cho'Gath. Her vision on her W is more of a minigame to the enemy team, as killing it will let Kalista figure out the pathing of an enemy (similar to an enemy killing an out of vision Rek'Sai Tunnel), while being revealed by it is no better. All this lead to her being a favorite of scripters who were able to dodge 100% of the spells thrown at her between auto-attacks and Q resets on jumping and landing perfect Rend (E)'s on champions/minions while having 0 map awareness and being dominated by the likes of Vi, Nautilus, and Maokai with point-and-click-CC. Much of her potential had to be toned down because when played perfectly she is nigh untouchable, and she's one of the few ADCs that "brings the engage." With that being said, however, a SINGLE slow will drasticalily reduce the quality of Kalista's life, and make her a mostly sitting duck until it wears off if she doesn't have the damage to face-tank and kill you immediately.
I feel that for both of these champions a few Quality of Life changes would drastically help them out. Azir is extremely mana intensive but is designed this way to prevent him flinging out a small army just to CS and threatening the lane the entire phase - if you ever face him in URF be prepared for SHURIMA. Kalista is a high-skill-cap ADC that excels in kiting and requires her Rend to kill a target to refund the mana cost (it used to be 2, IIRC). She also requires her Oathsworn to attack a target at least once to proc her W's passive, which requires great coordination amid a teamfight to utilize properly.
was supposed to be a "late-game hyper carry" but instead hes a massive lane bully against anyone that didn't flat out out-range him. Hit hits a huge mid-game powerspike with just 
and still scales insanely well into late game.
was the hands down most despised champion when she was meta because of her 1000% bullshit passive that completely invalidates melees + slows and outright unfair ult synergy with champs like 
was supposed to be a ''late-game hyper carry'' but instead hes a massive lane bully against anyone that didn't pick 
or 
. He hits a huge mid-game powerspike with just 
and still scales insanely well into late game.
