they don't have to be played in the final, they have to be one of the player's iconic champions or something like that like faker got zed, even though apparently he had asked for ahri or something like that
Technically they have to be the picks of the final game during finals However as pointed out there is overlap in previous team skins, so who knows
*hint* *hint* it was a punchline suggesting vel'koz is more suited to the support role than mid *hint* *hint*