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Child of Alara

Blue/White
Shield of the Righteous -What's better than Vigilance on a creature? How about Vigilance and keeping any one bold enough to retaliate tapped down until their next combat step. Will obviously see pairings with White with so many good cards Ironfist Crusher , Thoughweft Trio , Cenn's Tactician , Valor Made Real .

Wall of Denial -Will likely be the goto wall for UW, since it has more than double the toughness of Fog Bank and Angelic Wall . Shroud is just the icing on the cake, insuring that you stay well defended until the next global sweeper spell.

Blue/Black
Mask of Riddles -The new Sleeper's Robe will definately see play since the original was played quite a bit even though it set you up for a 2:1 card disadvantage. Mask of Riddles doesn't suffer this same fate.
Mind Funeral -An interesting card that some MP groups will toy with. Quite powerful against that player that always is playing land thinning cards, or maybe someone will use it against themselves to achieve threshold or access to other useful cards in the graveyard. Interesting but not powerful.
Nemesis of Reason - will take the place of Szadek, Lord of Secrets since it does what he does so much better. Triggering after declaring attackers makes NoR a threat to everyone, and it is much easier to cast.
Soul Manipulation -good counterspells are hard to find for MP. Any counterspell that nets you the card that you lost isn't half bad, {and a common to boot}

Black/Red
Breath of Malfegor -alongside Sizzle and Syphon Soul , this card is efficent pidgeony burn. As an instant {and another common, yay}, you can cast this on turn 5 at the last players EOT and follow up on your turn dropping a land and casting double Sizzles/Syphon Souls. Gets the obviously good for MP award.

Deathbringer Thoctar -Already mentioned by Kelly Diggs. Although BR is one of the more difficult color combinations to play in MP because of it's susceptibility to enchantments and because its colors usually burn through resources so quickly, this card will definately see play.

Lightning Reaver -Would take the obviously good for MP award except that with 3 toughness, and a constant threat to all your opponents, he is unlikely to ever untap. Good, but like Teferi to much of a nusiance to everyone to survive long.

Terminate -A great common reprint, pick up a set before they are all gone again.

Red/Green
Dragon Broodmother -because dragons are cooler than Saprolings.

White/Green
Mage Slayer -better than Fireshrieker on the offense. Since it triggers on a creature attacking, any opponent with less life than the creature targeted with the equip cost is going to need instant artifact/creature kill.
Behemoth Sledge -will see play since Armadillo Cloak sees play, will become the poor mans Loxodon Warhammer .
Dauntless Escort -will spawn a new wave of decks that don't worry about over extending in MP.
Qasali Pridemage -will replace all those disenchant spells/enchantments in WG decks.

EnemyColor
Tainted Sigil -might see play alongside some cheap burn Pox , etc.
Lord of Extinction -will see play and will be a constant annoyance with people stopping to ask "how many cards did you just put into your graveyard?". Play this and Chaos Moon and have fun counting instead of playing MtG.

3+ color
Mayael's Aria -The new I win card.
Flurry of Wings -Alpha strikes against the wrong player could spell certain doom for the aggressor.

As for personal favorites, any of the commons I listed {I'm a bit of a pauper mage}