Aye, there's something to simplicity - in my own ruminations on how I'd run it, I kept going back to simpler and simpler challenges (i.e., "improve the fort's military" -> "fastest to train a legendary fighter" -> "whoever gets the dwarf with the highest kill-count wins"). Ideally, the challenge itself should be simple, to allow multiple ways to accomplish that goal, which is where the fun and variety comes in. Switching between "fastest to do X" and "most X by the end of the year" is a good way to change things up as well.
Previous rounds should be taken into account. The weapon-smithing round is one that sticks out in my memory. It was tough but fun, but was pretty same-y because the Vesh statue round had already setup magma smelters (and meltable statues nearby).
Likewise, how easy it is for the players to check their progress is important. The new happiness bar (like the DFHack happiness bar) is a great example. A round where the goal is to make as many dwarves blissed out as possible is easy to keep track of, because the info is always on the screen. The booze round benefited from this slightly as well.
Also, just for playability sake - I think there should be some sort of housekeeping round every once in a while (either done by the gamemaster, or done as an actual round) just to clean up the FPS, the happiness, provide food, etc.
Either way, I'm very happy to run a Numbercurse 2 - even with a new host of players.