The Stoopid Unikorn
Spiciest of Guacamoles
I always thought Spirits were just Sakurai going all in with the Stickers idea from Brawl.I don't really see the link between spirits and Persona honestly (although I'm not the biggest Persona fans, only played 5). I think the modes was influenced by Marvel vs Capcom 3 Heroes and heralds modes. If you haven't played the game, it's a mode where you collect cards and each cards have an attribute and you can make a deck of 3 cards to have 3 bonus abilities on your team and fight other teams to collect more cards.
I guess I can see the similarities with HvH though.