In Magic: the Gathering, you can make your deck as big as you want, but no smaller than sixty cards. Everyone makes a sixty-card deck, in other words everyone builds the smallest deck possible. Adding extra cards to your deck reduces the odds of drawing your best cards, so no one does it. So having the option would be pointless, because no top-tier deck would use more than thirty-nine cards.
The reason Scion’s first wish and sphere of darkness are three-ofs in all Vetruvian and Abyssian decks is because they give a solid effect while drawing another card for one mana, so they almost let you shrink your deck, increasing your odds of getting your best cards, while rounding out turns that have a mana left over and giving a small (sometimes not-so-small) positive effect. They’re actually a lot like bloodbound spells in those last two regards, but bloodbound spells don’t also get you closer to your best cards.
I’m voting no in the poll just because I can see starry-eyed newbies shoving as many cards as possible into all of their decks, unknowingly hurting their own chances of winning.
PS: Although it might be interesting for them to change the mandatory deck size from thirty-nine to forty-five or forty-eight for a single patch, just to see if there are any notable changes to the meta or people’s deckbuilding. Not that they’ll ever do that, just a thought.
And now I’m wondering if one of the alternative game modes they’re making has a bigger deck size or only lets you use one or two copies of each card.