It would be cool if anyone can go through their match history and know whether the Kayn player transformed into the Shadow Assassin or the Darkin without having to go and watch a replay of it.
Sire Hippington4 voteswould also be nice to have them differentiated on their api so websides like op.gg can get different stats for rhaast and kayne. Them shareing winrate and so on feels rather wrong, it might also make balanceing each form hard if they don't get seperate data.
DrCyanide1 votesWithout an API change they can't display it on any website, their own or on third party ones.
They might be able to make it so that the same champion has 3 different Champion ID's, one for Base Kayn, one for Shadow Assassin, one for Darkin. This would still put some extra effort on websites trying to track stats, as there'd be 2 unbannable and unpickable champions that still have win rates and one champion that's picked but rarely finishes a game, but it might still be the easiest solution.
Sire Hippington1 votesMost websides offer different stats for different positions, based on lane proximity, which also have the issue off shareing the bannrate So if they can do that, they also should be able to offer stats for the two different paths for kayne, provided Riot's API does gives the information about the chosen path in each game.
DrCyanide1 votesThe way most sites generate that information is based off of what the API tells them. For each player the API gives a Lane (Top, Mid, Bot, Jungle) and Role (Solo, Duo, Duo_Support, Duo_Carry, and None). As you can tell, these are not what you got in champion select, but like you said they are based on where the person is in the first few minutes of the game. I've had several ranked games where the Riot API mistook me for "Jungle Xerath" because I had to bail my Jungler out on an invade. Some websites counter a bit of that by making guesses (this Xerath didn't have Smite and didn't kill any Jungle camps, he probably wasn't Jungling), but that's still where most of the data comes from.
The thing with adding an additional field like 'Evolution' (or whatever you'd call it) is that it would show up for every champion, even if it wasn't used. The API still has fields for Dominion (such as their score) that are saved for every single game that's not Dominion because that's how it works.
It's for that reason that I think the Champion ID field would work better. Champion ID is something that already exists in the API for every player, meaning that you don't need to make a new field that shows up 10 times in every single match. A change could be made to the Champions endpoint to indicate if that Champion ID was pickable or not and if it evolved from another champion, which is similar to how the Items work.
Another alternative would be to use the Timeline data to have an Event for Kayn evolving. This would require an additional call on all Stat sites for games involving Kayn, but would work as well.
MohawkBird2 votesNot a bad idea, but more often than not you can tell simply by the build path that they choose. Shadow Assassin is lethality and AD oriented, while Darkin might see more of a bruiser type build.
Karn Bishop1 votesYou don't need to watch a replay currently the answer is always darkin unless they are bad, nobody would give up %hp healing + %hp damage on top of an aoe 1 second knockup in exhange for a slightly longer poke skill and 5 damage post mitigation, unless you ulted after the autos then a whooping 10 dmg.
Brain Errör 4041 votesYou're missing out on fun factor and the fact that the champ is new... people are gonna experiment.
Camulos1 votesAlso, in the Assassin form the up to 44% healing becomes up to 44% bonus magic damage (for the first 3 seconds, but that's really all you need), if the team-comp already has fighters or tanks, it can be better to choose the Assassin and delete their back-line from a wall.
Problem is, most people seem to think the Assassin is supposed to play like 
Zed or 
Talon, playing him like an AD 
Evelynn (flanking, using the ult to avoid damage etc) works much better.
Sun Wu Ryuumoku1 votesPls don't make these assumption when u don't understand the champ (even the game).
I Love Thresh1 votesThat feels like it would be a good addition indeed. Meanwhile though, it's probably possible to determinate what form was picked through itemization
Khadaver1 votesUnless they decide to go lethality darkin [slayer-jinx-wink]
AirKingNeo1 votesDuskblade -> Bruiser is pretty decent on Darkin mode, because Duskblade is broken.
Chimp Joker2 votesI dont like to leave assumptions on some things. If riot can give out so much information on a game, I do not see why they can't just tell us whether he transformed into one form or the other.
Sun Wu Ryuumoku1 votesIts okay if u look at an profile... but i personnaly can't read information on stat web. I mean go on lolalytic and try to perceive what's best on Kayn or Raast... Good luck
General Esdeath 1 votesLike a counter? Because they can swap.
AwesomeGuyDj3 votesonce his form is chosen he's locked in.
General Esdeath 1 votesI thought, from the champ spotlight, you could swap, but I realize now it means you can unlock one form but pick nothing and choose in X minutes.
AwesomeGuyDj2 votesYeah, say you unlock blue form, but you dont want that. You want red, you can wait 4 minutes instead to choose red even though you didnt collect all the essence for it.
BLACK REALM GOD1 votesi agree. in player build it should show the passive as well and then when the player transforms it shows with transform they chose. it'd be cool to also show eggnivias egg activations and zacs blobs.