I had planned to wait until it was finished before I commented, but after thinking about it I changed my mind. While there are plenty of things I could comment about the mechanics (specially the blanks), I will just raise a few points:

Instead of a whole banned spell list I'd recommend just setting a guideline (the likes of unhinged, unglued and mythic rares/planewalkers) and keeping card acceptance a per-card thing. As there are plenty of cards that due to the way they behave within the system become even more powerful. For instance [url=http://www.blackborder.com/bbcart/images/prods/Magic-The-Gathering-Worldwake-Death-s-Shadow.jpg]Death's Shadow[/url], which is a tournament legal rare card and equals to a one mana cost, 8/8, 16 life creature if the player keeps 5 life points.

About combat: supposing you kept the dice combat of the original system (which if you abolish will make combat weird if monsters get life= 2X toughness) and direct damage spells follow locked combat, I conclude Garth is a pretty lucky fellow to kill the Hulk. As both roll five dices (5 from the spell and 5 of toughness) and the Hulk has 10 life points, Garth would need to roll at least >15 (as the Hulk will at least roll 5 on its defense and damage equals the difference between values). Especially so, considering how the Hulk has to roll around <20 for Garth to have a chance of success (a 20 roll would require a 30 for Garth). While possible, it’s a considerable risk and makes a rather strange combat example (yes I know this isn’t the real point of the example, but my point stands unless I miscalculated something).

Lastly, about player stats and breaking the game:sun : Supposing that when the players obtain their first color they acquire 5 build points, or even later, that’s enough to make an almost immortal character. Just use all those points in toughness to raise it to 11(11d6 = minimal value 11; maximum 66) and unless the character gets really unlucky against a strong attack, they are essentially untouchable. Even more with defender trait to get Reach. While I doubt anyone would make this, I would recommend limiting somehow the amount of power/toughness points someone may be able to acquire. 

There are other points, but for now I will leave it at this.