A mini-MMORPG would be cool, although it would really need to be able to sustain itself as a single player game too, since the userbase would most likely be fairly small (at least at certain periods of the day/year).

Indeed. The only time this ever happened was when we got slashdotted or stuff like that (when Portal Prelude was on Reddit front page we had close to 3000 guests online at once and when the Casio PRIZM was leaked we had close to 2000 from Slashdot). In other occasions, I'm fairly sure that many of the guests were bots. I never saw more than 40 users logged in at once before.

There are rarely 200 people online at once here