I strongly oppose a MTDG as a replacement to the SPDG. First off, multi-player games usually involve one goal: killing everybody else off so you can reign supreme. It is rare, IIRC, that the tech race progresses out of the middle ages or that any peaceful means of winning the game is pursued. And ultimately, the idea of the SPDG is that we are one big team attempting to win this game together, not four to six tiny teams trying to crush each other. And keep in mind that PBEM's usually go very slowly. We have had enough trouble with attrition over a few months (heck, I wasn't active in either of the two most recent DG's)--what makes anybody think that a multi-player game moving slowly and lasting perhaps over a year would be better for participation?
Now, I wouldn't mind if an MTDG were played as a separate game, like the CISDG or MSDG were. I might even participate to see how those turn out.
But please don't kill the single-player demogame.