I'd be really interested to see what Mearls would come up with if he was given free reign to design his ideal version of D&D - without having to take into account the market, WotC, other designers, etc. Just starting with a blank sheet of paper and creating the game that he'd want to play. I'd be really curious to see the result.
You might want to have a look at "Iron Heroes," which Mearls did most (possibly all, I'm not sure) of the design work on. I'm not saying it's his ideal D&D, and I'm sure his ideas have evolved a lot since then, but it offers some interesting insights into his approach to gaming. There's some definite Iron Heroes influence in 4E... though not as much as I'd like, to be honest.
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