Just a couple more bits from someone who very much appreciated the IV GotM back in the day, but has not (sorry) found much appeal (yet) for participating in V GotM. Obviously, the format works for the regulars, and I have no idea how much appetite there is for trying new things.
With those caveats out of way, I would suggest rotating only through Emperor/Immortal/Deity and dropping the easier difficulty levels out of rotation completely. I think it would be more interesting to have more people describing how they managed, maybe just barely, to get through the game. As it is now, the dialog seems to be only about fewest turns to victory. Which makes sense when half the games are at a difficulty level where the only way to make them challenging (for most players on the forum) is to get through them as fast as possible.
For someone who is challenged at King and new to the forums, finding the GotM at King might seem ideal -- but I suspect that they would not come away with much that they can learn from. And they are certainly not going to feel welcome with the competition being so focused on quick wins. That person hits the same wall if s/he looks at Prince or Emperor. So the GotM is never really very inviting to a new participant, no matter what their skill level, until and unless that new participant is a very good player.
In contrast, if the GotM games were always at Emperor or better, I think you could regularly (at least with a third of your games) attract new additional Prince and King players that would at least be able to dialog with each other about their experiences. Having every third game a Deity run would have curmudgeons like me participating regularly (even with the Emperor and Immortal games).
Can you guys track your statistics of new participates over time? I suspect they drift in and out at pretty constant pace. But, of course, it is more likely that I am just projecting my own prejudices!