What's a good set without walls?
Some of the names and whatnot may have been done already, so bear with me...
Black
Vile Wall (Uncommon)
Creature - Wall
2B
BB, Tap: Sacrifice Vile Wall to destroy target land. Play this ability only when Vile Wall is declared as a blocker.
1/4
Wall of Death (Common, Holy cheesy name Batman!)
Creature - Wall
2BB
If a creature is put into it's owner's graveyard after being dealt damage by Wall of Death, Wall of Death gets a +1/+1 counter.
1/2
Blue
Artificial Wall (Uncommon)
Creature - Wall
UU
U, Tap: Sacrifice artificial wall to return target attacking creature to it's owner's hand. Play this ability only when Artifical Wall is declared as a blocker.
0/2
Wall of Steam (Common)
Creature - Wall
2U
R: Wall of Steam gets +3/+0 and first strike until end of turn, play this ability only once each turn.
1/5
Green
Ironoak Wall (Uncommon)
Creature - Wall
2GG
Tap: Sacrifice Ironoak Wall to add GG to your mana pool. Play this ability only when Ironoak Wall is declared as a blocker.
2/5
Wall of Life (Common, and another cheesy name!)
Creature - Wall
GG
Tap up to 2 creatures you control. Wall of life gets +0/+X until end of turn, where X is the combined toughness of the creatures tapped in this way.
0/3
Red
Exploding Wall (Uncommon)
Creature - Wall
RR
RR, Tap: Sacrifice Exploding Wall to deal 3 damage to target creature or player. Play this ability only when Exploding Wall is declared as a blocker.
1/2
Wall of Breath (Common)
Creature - Wall
2R
RR, Tap: Wall of Breath gets +2/+2 until end of turn.
1/4
White
Glass Wall (Uncommon)
1W
W, Tap: Sacrifice Glass Wall to prevent all damage that would be dealt this turn. Play this ability only when Glass Wall is declared as a blocker.
0/3
Glittering Wall (Common)
2WW
W, Tap: Prevent all damage that would be dealt to Glittering Wall this turn.
0/6
And how about this for a new mechanic? While not exactly worthwhile in the beginning, it can prove to be pretty effective.
Slow
KInd of the opposite of Haste, the creature would have summoning sickness for 2 turns instead of 1.
eg.
Portly Mage
Creature - Mage
UU
Slow
Tap: Portly Mage deals 1 damage to target creature or player.
1/2
"The Portly Mage's girth brought him slowly to the battlefront, but once there he proved his effectiveness."
And if this post couldn't get any longer...
Songs for each color...
Black
Song of Hatred (Rare)
BB
Enchantment
BB: Sacrifice a land to destroy target land.
Blue
Song of Whimsy (Rare)
UU
Enchanment
UU: Discard 2 cards and remove them from the game to counter target spell.
Green
Song of Friviloty (Rare)
GG
Enchantment
GG: Pay 2 life to give all creatures you control +2/+2 until end of turn.
Red
Song of Destruction (Rare)
RR
Enchantment
RR: Deal 3 damage to all players.
White
Song of Harmony (Rare)
WW
Enchantment
WW: Tap target creature you control to gain 2 life.
And that's it for now, and I have plenty more ideas... I don't think anything's broken, it's hard to have a broken wall right? I tried to balance the songs, but I'm not sure if I over or under did it. Sorry for being so long...
D