I think your proposal sounds very interesting. I, for one at least, would be interested to read what you come up with. The thought of using minifigs and brickbuilt dungeons/encounters for an actual D & D game has been on my mind for quite a while now--I suspect it would be very labour intensive, but a blast at the same time--but alternate rulesets would also be fun to explore.
On a tangentially related note, have you checked out Brikwars
? I know that it's in the same genre as the game which you are proposing, but you may find it intriguing nonetheless.