Very impressive. I've thought about this type of thing before. Mind if i toss a few ideas?
First off, we all know D&D is a pain in the ass to learn how to play. So, to simplify everything, use the basic D&D engine: Reduce everything to small numbers. For example, dealing damage, when a character attacks another character, the player rolls a D20 (twenty sided die) and checks the AC (armor class), if it matches the number, or higher, it's a hit...ANYway, instead, make it simple. Perhaps use a 6 sided dice, or any simple method you can think of.
As for classes, start with the basics: Fighter, Wizard, Rouge, Ranger, Paladin, Cleric. Design those however you want. And for the Newgrounds theme, add classes like: Portal Guard, Dark Blammer, Spammer, Clock, Tankmen, etc.
I like your adaptation of Magic: The Gathering theroy. A good way to use this, would be for casting spells, or adding magical equiptment to your character. For instance, during a "battle", a player will draw a hand of cards, and use whatever cards they have, to their advantage. You don't have to limit this to magical spells. You can create "Fighting move" cards, for the melee combat types, like "slash attack" or "whirlwind attack", somthing crazy, i dunno.
There's a few ideas, i'll send some more soon.
