So lemme give this a shot. I think Tobi's right here, so I'll start with this:
1. The deck should be good without Leyline
2. The deck should be the nuts with Leyline in play
3. The deck should have use for topdecked Leylines, or not have a problem with dead draws
4. The list should include many cards that make the most out of a Leyline in play, but are also good without it
It seems to me that the most powerful cards to combo with Leyline, which are not dead on their own, are discard spells. Leyline+Raven's Crime in particular is a pretty nasty soft lock if you can keep the lands coming. So start off with:
4 Leyline of the Void
4 Duress
4 Thoughtsezie
2 Raven's Crime
Late game Leylines are not useless here, since you can establish the lock whenever. But we need a way to survive the early game; even with full power it will be hard to land a Leyline before the opponent can disrupt you. So how about the unpower 8.
4 Null Rod
4 Chalice of the Void
We need some way to keep the lands flowing once we are established, and Life from the Loam sounds best.
1 Life from the Loam
And some way to win that likes to see the other cards we are playing:
4 Dark Confidant (beatz!)
2 Vampire Hexmage
1 Dark Depths
Round it out with some tutors:
1 Demonic Tutor
1 Vampiric Tutor
1 Living Wish
3 Crop Rotation
And a land toolbox since we're packing Rotation, and Crime likes to see lands:
1 City of Brass
1 Maze of Ith
1 Strip Mine
3 Wasteland
1 Cabal Pit
And a manabase:
3 Verdant Catacombs
3 Polluted Delta
1 Misty Rainforest
2 Underground Sea
2 Bayou
1 Tropical Island
2 Swamp
1 Forest
1 Island
And, finally, sideboard:
Living wish toolbox:
1 Dark Depths
1 Vampire Hexmage
1 Wasteland
1 City of Brass
+12 metagame slots
So, this deck survives the early game with Chalice, Rod, and Duress effects, then seeks to either combo out Dark Times style, or establish a soft lock with Raven's Crime + Leyline of the Void. The Leyline isn't a dead draw if it doesn't come down immediately, because hard casting it is value-added in mid game, too.
What do you think?