All these mage combos! I feel like mage already has among the most synergies in the game. Mage/legend, mage/restoration, mage/destruction, and mage/alliance are already good synergies.
Some keywords are comparatively neglected, like brigand. Amping up the brigand/undead synergy could be cool as a way to diversify undead decks (pure undead for all-around goodness vs. undead/nature for raw power and rare-drawing, undead/brigand for control).
Also, I'm a big fan of one-off combo cards that don't dictate a whole new deck-building style, like Stone Minotaur. I just think they're fun, and make the game more interesting for various reasons.