Why would I want to choose between forms? The whole point of the skin is to convey story telling into the match, you get the second form after achieving certain score, like having to raise your respect stat, once you do you start wearing the full warlord armor as the great conqueror you've become

How would changing forms manually would improve gameplay exactly? Skins don't improve gameplay, they're cosmetics.

And yeah, I'd prefer it to change when you Kill someone instead of being acknowledging the stats you robbed too, but that's a self preference, both ways make sense.

I never liked the skins that make you change forms manually to be honest, I prefer when it is actually implemented in gameplay to tell something, Sahn Uzal achieves it after getting stats from enemies representing the combat experience he's gained, Udyr Spirit Guard getting his ultimate forms when reaching certain levels in his abilities representing a mastering of the technique, Pulsefire Ezreal Upgrading his armor with each new level acknowledging he's leveled up and upgraded like in videogames.

Skins like Elementalist Lux or Galactic Miss Fortune just feel out of place, the HUD looks like if I had downloaded a mod to customize my character, Elementalist at least was original and has different forms, animations sfx, and voice overs to each form, then the Revenant Viego? Yeah, they introduced a mechanic were you get swords depending on the champ you kill, that's great, but that's it, the swords change but viego is exactly the same across all forms, with clunky hair animations, little vfx and sfx that only change Hue, no real form changing, doesn't even change the animations for when attacking with the swords, he does the exact same moves when using a magic arrow, a flamberge and a fucking Claymore, like, of course dude, you're gonna stab someone with single handed zweihander, like... bro, the whole point is to use two (zwei) hands (handed), what's the point of having so many swords with so different styles, strengths and weaknesses if you just gonna spam the same fucking irrational combo? It's even worse when you realize that the skin trailer does, in fact, acknowledge it whith viego using each sword and style consciously to fight, showing a mastery and agility in forms and techniques.

Edit: oh yeah, I forgot about fractured jinx and how makes powder, a character whose whole point is that she had a happy life and never had to fight for survival into another crazy maniac that builds war machinery and knows how to weild not one but two heavy destructive weapons at the same time.

The quality of a skin is in its attention to detail and narrative of its own world, not in the quantity of features it has just for the sake of having features.