Champions start out as a general idea,sometimes a cool gamemechanic(tahm was created because they wanted a champ who can eat others),sometimes an archetype that they want in the game(yasuo was made because they wanted a more traditional samurai champ),sometimes as a sketch(like the jinx example of FeBeast) and sometimes because they want to make a certain role of champion(Taliyah was made because they wanted to make a new mage)
And overall I doubt that Lore is really limited by the gameplay.
I mean take Ekko for example.
In the game he can rewind time only for himself and he has a huge cooldown on this ability.
In the lore,he can rewind time over and over again and he rewinds everything.I mean in his comic,he travels ~100 times back in time to the same spot,that wouldn't be possible in the actual game.
Likewise aurelion sol seems to be roughly the size of a mountain in the lore with the ability to destroy the whole planet if he wanted to do so,yet he can be killed Teemo in the game.