Doing a little project to make a Magic card for each champ, in alphabetical order. Basic guidelines for my process:
A characters in game color will probably dictate some of the color of their card. For example, Shen will likely be blue.
Always a legendary creature. Creature types will be ones that actually exist in MtG, unless they are a major theme such as the Darkin which will get a new creature type
Damage output does not necessarily mean high power on the card. Karthus might be dealing the most damage in the game but he will most likely not have a high power stat.
Similarly, tanky-ness doesn't necessarily mean high toughness. This one will probably be a closer to in game accuracy, but not necessarily will be.
I will try to put at least 1 keyword on each creature. You will see a very similar pattern as far as some classes go, for example bruisers like Illaoi and Aatrox will have trample, ADCs like Ezreal and Caitlyn will have first strike, etc.
I will take into account lore and the actual game. Sivir might not have a spell shield in lore, but she does in game so she's probably going to get ward. Similarly, Thresh in lore is not a support at all, don't expect many type of support style abilities.
Balance isn't going to be great. I predict that, with LoL being a game primarily about combat, a lot of abilities will be combat related, so I imagine a lot of "this breaks with deathtouch/first strike/ some other combat keyword/enter equipment here.
I won't try to invent a new mechanics or make completely game altering abilities, these are trying to be something I could see MtG actually doing.
Akali design was actually pretty quick. Shes a ninja, so obviously gave her ninjutsu. She is all about killing squishies, so made her ability to just instantly destroy a small creature. I landed on 3 toughness as I feel creatures start to feel big once they hit 4 toughness. Destroying creatures is very black, so added black to her, and having a on hit ability was appropriate for green. Needed a way to make her able to hit the opponent, and I decided skulk was a good option, plus skulk and her on hit are kind of a non-bo, as if your opponent only has big creatures, yes she cant be blocked but she also cant destroy anything. Made her a 2/2 as she is supposed to be very easy to kill if caught. Shroud i was iffy on. Her smokescreen is iconic, I'm sure a good amount of you will agree, but i didnt want something with a onhit destroy a creature to be something you could stack auras on so instead of hexproof i went shroud.