I'm a big melee fan and my favourite champion is Garen so I can totally understand where you are coming from with this.
However, I disagree with you; not on everything, but a majority.
- Kalista's potential mobility is absurd but passive is not bad by design. Agreed
- Passive should've gone to a melee. Disagree
- Range gives +1 advantage, so they don't need another +1 from mobility. Disagree
- Melee champ with this passive could mind game, have better risk/reward, etc. Disagree
a) Passive should have been given to a melee instead
Two things; First of all, this isn't going to solve anything or make the situation any better; Second, we already have a melee champion that does this.
Why won't it solve anything?
- The melee roster is practically saturated with gap-closers already (47 out of 68 melee champs, 69.1% - *MS streroids not included, form-shifters included).
- This in turn has already led to the proliferation of slows (83 out of 121 champs have abilities slows, 68.6% - *For simplicity, we'll ignore the fact that some champs have more than 1 ability that slows).
Just looking at those stats makes me feel sick. In case any of you don't quite get why those stats are a big deal, I'll explain it briefly.
League didn't use to have a lot of gap-closers. As they became more common, the idea was "Oh okay, we'll add more slows to counter the mobility.". Here's the thing, slows don't really counter dashes; slows hard counter MS steroids. This is why picking a champion with without a dash or just an MS steroid feels much worse than it actually is.
So trust me, adding another melee with a gap-closer isn't going to solve jack shit. It will just keep the cycle going.
We have a melee champion that does this already?? Wha...?

Yasuo moves around more than any other melee. His Q is basically auto-attack that can't be cancelled just like Kalista. He can dash around a lot and even E>Q to deal damage while moving. Kalista is the ranged parity of Yasuo without the same mobility limitations. However, if we ignore the other potential problems with her kit, it is not her mobility that makes her OP, it is her Attack Speed.
Yasuo can't just dash as he pleases unless the circumstances are right. Even in a duel, he needs to maximize auto-attacks + Q to be effective. This is why it is okay for him to have a 0.658 base AS and end with 1.016 AS naturally. It is not okay for Kalista to start with a 0.648 base AS and end with 0.978 at 18. Her dash distance can already be improved twice by buying then upgrading boots. She does not need her innate potential mobility amplified by naturally high AS values, rather she should be forced to invest in it.
Azir is a great example of what I'm trying to get at. Azir's power is directly impacted by the amount of AS he has and since it is naturally gated (2nd worst base AS at 0.6 and 5th worst AS at 18 with a measly 0.753), he is forced to acquire it somehow to maximize effectiveness.
b) Range provides an innate advantage, so ranged champions shouldn't get mobility as icing on the cake
I would have agreed with this but sadly, 1) there are more Melee champs with gap-closing power than those without (even worse with MS steroids included) and 2) the sheer amount of slows make it alright for the occasional ranged champion to have mobility. The crucial part would be to keep a good balance of mobile and immobile ranged champions.
So let's do a stat comparison:
Shapeshifters included, Kalista excluded
- 16 out of 56 ranged champs have gap-closers (28.6%), 8 of which are Marksmen
- it becomes 34 of 56 with MS steroids included (60.7%) 17 of which are Marksmen
Shapeshifters and Kalista included
- 17 out of 57 ranged champs have gap-closers (29.8%), 9 of which are Marksmen
- it becomes 35 of 57 with MS steroids included (61.4%) 18 of which are Marksmen
Would you look at that, 50-53% of both categories are champions classified as Marksmen which means the rest are Mage/Supports; can't say Riot is unfair or playing favourites. Seems pretty balanced as far as ranged with gap-closers go. When we include MS steroids, we start seeing a much more familiar picture; just like Melee, there are more champions with mobility that without.
This is indeed a big no-no, but considering melee mobility distribution and the prominence of slows, it isn't nearly as big of an issue. There are way less gap-closing ranged than melee. Even with MS steroids included, ranged still lose by almost 10%.
All this hasn't even taken into account the crapton of CC melee champs bring and the increasing number of Tenacity-immune CCs (knock-up/knock-back/displacements).
c) Melee champion with this passive make plays, has better risk/reward
The passive isn't bad, even you admitted this. The problem lies with everything else that affects the passive.
Fact is, anybody (ranged or melee) with this passive would be able to make plays and mind game the opponent; anybody capable of dealing %HP damage will have balancing issues; anybody with sustain/burst/poke can dictate engages/disengages well.
There is risk/reward for both melee and ranged when auto-attacking and spending cooldowns. Sure, ranged has an edge but it is ridiculous to say that melee are helpless.
You can't just tunnel on this passive and think it's the most OP thing ever. There are several things at work here (attack animation, attack speed, missile speed, dash animation). There are ways to tune it. Even with her high base AS, Kalista doesn't just auto-kite; you need bloody good input that is both quick and precise. She isn't some no brain, right-click to win champ. She isn't immune to stuns or gap-closers. She is single target damage with shitty waveclear. Her ult relies on both her and her soulbound not being complete idiots.
She may seem like she has balance nightmare written all over her, but honestly what the fuck did you expect? This is a CertainlyT champion. It's supposed to be new and exciting first, balance later. So far it looks like mission accomplished for the new and exciting part, give it two weeks or so and we'll start seeing some devs chime in on balance once she's had some real playtime. It's also preseason right now so we all need to suck it up cause there are going to be things which are bloody OP.
Mid-late game Kalista can move left and right more than a fish out of water, yes, that's her thing. She'll probably see competitive play, maybe get nerfed then buffed again. Who knows?
Just try to not bandwagon on the passive being broken. It hasn't even been a week.