I don't like the idea of class-based suits because of overlap and an unclear distinction of spell origins.

If I did a Deck of Spells then I would base it by school or type.

Hearts: Enchantments, mind-control
Diamonds: Abjuration, defenses
Spades: Transmutation, buff magic
Clubs: Evocation, attack spells

However, I'd also try to stay true to the Tarot style of the Deck of Many Things. In that sense, the Major Arcana should be major spells. Likewise, the royalty cards should be stronger than the numerical ones. They would also carry different suit names: Swords, Cups, Wands, and Coins. Incidentally, these tarot suits also bear a striking resemblance to the early four classes -- fighting man, cleric, magic-user, thief. So another way to go about it is having Swords be offensive magic, Cups be divine magic, Wands be arcane magic, and Coins be social magic.

Theme is just as important as mechanics when it comes to designing items. When players see your work and notice the minor details that make an item consistent with logic, they will appreciate its authenticity more.