Eric's comments about sower of temptation over in the first pick thread have got me thinking about. I've been trying to find a control magic varient that doesn't feel horrible (I think the right answer is legacy's allure), but came across
abduction
Now, back in the day I wouldn't have touched...
I really like it actually. It's less about breaking the symmetry, but if you're in a ramp deck it provides a roadblock that helps get you to the game stage you want to be at. Also good to sacrifice. It helps both players have an effective game, which helps the overall experience.
These people were at the UK Games Expo, and I seem to remember them having a 'magic' box that looked suitable for cube:
http://www.basicallywooden.co.uk/
How about, for graveyard effects and synergy
starfield of nyx
You already have
vessel of ephemera
Vessel of paramnesia
Seal of removal
Font of fortunes
Dead weight
Seal of doom
Seal of fire
Vessel of nascency
Narcissism
Saproling burst
You could support further with
monk idealist...
Spitballin'
noyan dar, roil shaper links spells matters with lands.
Some more 6+ cc creatures for fierce empath, maybe cyclers? Deadwood tree folk. Maul splicer. Spitebellows.
ugin's insight and nissa's revelation for high cc shenanigans.
Have you lost the spells matter from blue? There's only rise from the tides, vexing scutter and mnemonic wall as hooks.
How do you feel about
karametra, God of harvests
Also
not forgotten for grave pulse decks.
dauntless cathar and nearheath pilgrim for self mill and value.
Other lower power land cards
kor cartographer
Endless horizons
Ruin ghost
I've re read through the thread and there are a few leaps where some of the design principles have been left behind,most I'm not sure where we are on some things now. Not a criticism, but my thinking might be out a little from where you are.
White is always the problem child with synergy cubes...