Given the card rarity is UR, if it is available outside of an event in the future, most likely it will be either an event drop card, card trader or future Dark Signers level up reward (level 40 or 45 etc).
Unless the rarity changes to SR/R, then it might be released to a box card.
I don't think this card really deserve to be a future box UR though.
I can bet it will be available through a roaming event by defeatung whoever 15x. If they choose to give out just 1 copy, then it could be given as a reward like they did before with Winged Dragon Of Ra (alternative art)