howandwhy99
Adventurer
IMO, a DM cannot play a PC. It's like trying to answer a riddle you told yourself (something I've been told is at the heart of storygaming). Maybe a better example is trying to play Mastermind(TM) against oneself. If you set the colored pegs up behind the screen, how can you also play that game?To play a bit of devil's advocate: What if only some of the players are part of drawing up any given part of the maze? Can it still be a sandbox?
For a real world example: Unless I'm mistaken, Rob Kuntz continued to play in Gygax's Greyhawk campaign even after he became a co-DM and began developing his own portions of the campaign. I'd argue there's nothing in that arrangement that would be inimical to the sandbox.
I don't know how Gary did it and I'm not looking to emulate his game, but IIRC Rob played in the parts and dungeon levels of Greyhawk he did not have a hand in making. And vice versa for Gary in El Raja Key.