My modern cube. I have no idea how a modern cube would work so I copied a list and made a bunch of changes. Suggest me cards!
Ninja is on the chopping block for Thassa btw. Spending so much money...
I can't spend too much time analyzing it, but...
Don't feel like you have to be married to symmetry necessarily - especially when it comes to planeswalkers. You don't necessarily need two of each color - feel free to mix and match.
Cards that it feels like you're missing:
* Nicol Bolas, Planeswalker
* Birthing Pod
* Voice of Resurgence
* Garruk, Caller of Beasts (a sweet way to cheat cards into play)
* Tooth and Nail (possibly even Progenitus)
* Intangible Virtue (or some other Anthem effect for white).
* Keranos, God of Storms
* Eidolon of the Great Revel
* Riftwing Cloudskate
* Shadowmage Infiltrator (it was time-shifted, so it's Modern-Legal)
* Banisher Priest (I think I like this over Fiend Hunter since it attacks for more, and you aren't running instant-speed blink shenanigans).
Cards which seem like they don't have a place
* Dryad Militant (take it out of Selesnya and put it into white if you want to keep it; it's chewing up a valuable gold slot for something boring). Turn this into Voice of Resurgence.
* Flames of the Blood Hand
* Blood Scrivener (just not good enough)
* Steam Augury
* Legion Loyalist
* Deadbridge Goliath
* Vines of Vastwood
EDIT: Added few others