I know that Ashling the Pilgrim plus 99 Mountains is a fun meme, but it's just so meh otherwise
Black tends to work best for milling when paired with blue, because its small attacks do advance a milling strategy
The set was originally teased on February 27 (Pokmon Day) in a brief, very vague aside during the lackluster Pokmon Presents
IIRC in Yugioh there is only a certain number of Blue-Eyes (from the show?), so you could put a rule on the card 'there may only be X Blue-Eyes on the battlefield at any time', Star Wars CCG used to have permanents that you could have 2 or 3 copies of in play, in addition to stuff you could only have 1 of, the concept would work fine here too, only this could care about both players' boards
Relive the series with iconic scenes, collect cards featuring yo
Creating three tokens for 3 mana is going to be good and will trigger a lot of the other cards in your deck nicely