Heres my favourite
The game automatically draws for you at the start of your turn, and when you play a card or a move that lets you do so, it highlights which cards are playable from your hand and where they go on the board, which is super helpful when you're just starting
Doctrine: Communion of life and love
Valiant is an ability word that appears on many Mouse creatures and highlights abilities that trigger whenever one of those creatures becomes the target of a spell or ability you control for the first time in a turn
Spectral Sailor is your best one-drop, and probably would be even if it was just a textless 1/1 flyer with flash, but its card draw ability is dramatically more relevant than you might realise at first, as it helps you pull ahead in card advantage in the games where your Curiosity-effect engine is broken apart
If we assume that each set will have serialized versions as 1/500, the odds will be that less than 1% of Collector boosters will have one