Well, you asked...
Here are the changes I'd make from the original decklist.
BLACK:
-2 Persecute
-4 Terror
-2 Rain of Tears
+4 Expunge
+4 Hymn to Tourach
Reason:
Persecute is neat, but it's almost useless without an early Ritual, and even if you DO get it off, drawing a second one is pretty useless. Rain of Tears...well, six LD spells won't do anything that four LD spells won't. Hymn to Tourach is cheap, random discard, and you don't even need a Ritual to make it work. Expunge is better than Terror in that it does the same thing for a piddly one extra mana, and if you end up not needing it (black player, Draw-Go with Morphling, etc.), you can cycle it for something you DO want.
Blue/Gold:
-2 Unsummon
-2 Seer's Vision
+2 Treachery
+2 Undermine
Reason:
Unsummon is a creature-only answer, and it's not a permanent one; if you don't have some way to prevent it, they can just summon the creature again next round, and you're right back where you started. Seer's Vision is neat, but the Coercion effect is mediocre due to its requirement of an initial investment of 4 mana, and Telepathy simply doesn't help you enough to be worth it. Treachery is a more permanent answer to your creature problems, and a resource for you besides. Undermine provides you with a little more counter power (and this deck could use some), and does some helpful damage besides.
Creatures:
-1 Alexi
-2 Somnophore
+3 Fog Bank
Reason:
Simply put, both of the creatures you had in here are very situational. Alexi can be nice, but she needs two cards in hand to do anything, and Somnophore can be stopped even by relatively small flyers. Fog Bank is the biz-omb...not only can it stop opposing Morphlings, but Avatars of Will, Serra Avatars, Shivan Dragons, etc. A creature that *will not die* during combat is some good, especially in a control deck.
Mana Sources:
-2 Dark Ritual
+2 Underground Sea (if you can get them)
or
+2 Salt Flats
Reason: Dark Ritual good. Dark Ritual also temporary. You need to have your mana sources constant and diverse, and dual lands are the way to go for that. Control decks generally need to land-drop at least the first three turns, and 24 land will allow that to occur the vast majority of the time.
Other cards to find room for:
Jayemdae Tome (card advantage)
Soothsaying (it's like GOD in this deck)
Lobotomy (for those pesky stains that won't go away)
