Neoxtreem

Does BTE count for 4 devotion for Xenagos since it costs hybrid mana?
Same question is relevant for other hybrid creatures for the two-colored gods.
This would be crazy.

Technically, he doesn't have red cards either (BTE can be played for GG). However, as mentioned in another comment, scry is very useful.

Talking about Theros, how about Nykthos? You will have lots of blue mana symbols on the board anytime.

How about splashing black for Zur the Enchanter considering the amount of enchantments you have.

As was said before, this is a very creative approach.
There are some cards with flashback you might consider (maybe for sideboard) that work well with this mechanic:

Ancient Grudge (sideboard for artifact heavy decks)
Conflagrate (Flashback: discard cards)
Deadly Allure
Increasing Vengeance (More spells, obviously)
Nightbird's Clutches (To be able to swing your opponent directly with Kiln or Charmbreaker)
Past in Flames
Wild Hunger (also very good combind with Kiln Fiend for example)

Anyways, these are just suggestions I found when looking for flshback cards. For retrace you already picked the most useful ones.

Have you considered -2 Wight of Precinct Six and +2 Avatar of Woe? I can imagine them being more useful.

Tome Scour would lower the mana curve a bit. I second that.

Have a look at Isochron Scepter. It works really well with all your 1 or 2CC instants.

How about Azorius Charm? Especially useful combined with Ashiok's +2 and -X.

I suggest removing Wall of Reverance because you will usually only have creatures with power 1 or 2 on the battlefield.
Take a look at Knight of Meadowgrain maybe.
Loxodon Warhammer would also be nice in here.

Interesting idea. You could try out how Ashiok works in here. I found him quite useful combined with Azorius Charm (flip creature on the library and instantly exile it).
Also Theros Elspeth is often used in Esper control.

Just return it to your hand. It's a pretty nasty combo with Agent of Fates. It might be interesting to playtest, since you have several spells to protect your Agent.

True, I also like the idea of having it in here. Would be epic combined with Nicol Bolas (not the plainswalker).
Dark Ritual into Entomb and Goryo's Vengeance. Opponent discards his whole hand first turn if this goes off.
Holy crap I just saw that Nicol Bolas has been reprinted in Time Spiral so it's also Modern legal.

Is there a reason for Aether Vial not being in your mainboard?

I know this deck was made before Theros came out. Your might consider the following two cards:
Purphoros could be worth taking a look at. In combination with Siege-Gang Commander especially.
Also Nykthos. That card is good in any monocoloured deck, incredible mana ramp.

How about adding Vesuva or Thespian's Stage? You can get even more Cathedrals or if needed you can also copy your dual lands.

I suggest adding some more card draw. Otherwise your vials will be useless when you can only get one creature each turn.
You might also consider adding a few bigger sliver for finishing the game. Your mana curve is so low anyways, and the mana slivers are good ramp creatures.
E.g. Essence Sliver for Lifelink (maybe sideboard), Megantic Sliver, Might Sliver.
Virulent Sliver would also be a worth trying out for alterntive win condition.

Slaughter Games is the card TheBear was looking for btw :)

I know, but it would be fun seeing how well this would compete against other modern decks. Just wanted to point out, it only depends on this one card to change its legality :)

I'm not quite sure to be honest. Maybe have a look at other mono black EDH decks.

I do agree with Soltur. Especially if you don't want to unleash the crackler, there is no reason to have him in there at all. He is only decent for aggressive matches imo.