Lately I've been having fun thinking about how my OCs would be translated on to Magic cards. It makes me think more about how I want that character to be, which is fun.
Not sure if Yamakoshi's card is balanced, an expensive, one-sided board wipe that gives you a hard to remove beater and an army for next turn.
I made this a few days ago and thinking about it again, I think I'd make the face-down creatures "Balloon Egg" non-artifact creatures that you can turn face up for their mana cost and gain the Balloon type.
Unfortunately MTG Cardsmith is down at the moment, so I can't make a new version.