Interesting! I can definitely see the appeal -- I actually just cut white from my uncommons cube (link points to my blog).
I'm especially impressed at you getting mill in there. At first I thought, "Mill in cube? Rookie mistake." Then I read the rest of your paragraph; using mill as a foil to card-draw shenanigans is clever.
That said, there are a few things that concern me, and I wonder if they've been issues:
It seems like there aren't that many decks to build -- like a lot of cards just fit into one deck. Aggro seems to be either Merfolk or Faeries/skies, for example, with little wiggle room.
A lot of the decks look like they require specific cards. There's no reanimator deck, for example, that needs one of ten fatties and one of ten ways to reanimate said fatty; instead there is a Tolarian Academy deck, a High Tide deck, some random two-card combos that are unplayable apart, etc.
It looks from some angles like the control and combo decks will basically just be the sorts of things you might see in a powered cube, but that the aggro decks will be much weaker (no self-respecting aggro deck in a cube plays Elvish Warrior). Have you considered dropping the average power level a bit, or powering it down?
Planeswalkers will probably not be guild-aligned, just like they were not faction-aligned in SOM. In that block every card except for the planeswalkers had a watermark; even though they opposed the phyrexians, planeswalkers are always on their own team.
We have been told repeatedly (and can easily see from what's been printed) that color balance at mythic is not an exact science. Planeswalker color is determined by creative, not design.
Shards block had a multicolor theme. There were three multicolor planeswalkers and two monocolor ones.
Sunstriker is the obvious choice. Right now we're looking like heavy white with one awesome blue card and one awesome black card. (Cower in Fear is fine but it's possible that we won't be able to support double black).
It's awkward that he's a Cleric instead of a Soldier for War Falcon.
I think it's got to be Chronomaton but Silvercoat Lion is also a consideration. Our pool so far is much better for BWu than anything else (I don't think we should be taking double-blue cards).
The only thing we have tying us to blue right now is Arctic Aven -- he's pretty mediocre if white is a splash, but splashing for a 3/2 flying lifelink seems pretty great in an exalted deck.
Sudden Disappearance W
Exile target nonland permanent. At the end of your next upkeep, if you didn't pay the echo cost, return the permanent to the battlefield under its owner's control.
To have this work you need to phrase it more like a Pact:
Exile target nonland permanent.
At the beginning of your next upkeep you may pay 2W. If you don't, return the exiled permanent to the battlefield under its owner's control.
Something I have to nitpick though: why are Traps and Arcane spells automatically off-limits?
There are complex cards there, sure, but are you seriously telling me something as basic as Cleanfall is going to be too complicated for a new player to get...JUST because of one extra word in the typeline.
Exactly. Traps are much more complicated cards than anything in the cube -- alternate casting costs are not something I want to subject these players to. Arcane is the other side of the coin, where the cards are mechanically simple but they have this subtype which has no mechanical or flavor bearing on the game, and only serves to be confusing. Since there is flexibility in which cards to include I chose to omit arcane cards entirely in favor of non-arcane equivalents.
If you want it to work at one mana, I think looting right away has to suicide the planeswalker. Something like (drawing from the original Jace):
Littlest Jace (M)
+1: Each player draws a card then discards a card.
-1: Target player draws a card then discards a card.
-2: Target player puts the top X cards of his or her library into his or her graveyard, where X is the number of cards in that player's graveyard.