It seems like what is wanted are quantifyable ones, Quotas or Size/Quantity measureable features.
If not, I have all sorts of ''special project'' challenges in my personal que I'd like to see tried out.
However, sticking with the numbers based ideas for now...
I know some (most?) are kinda lame, but I'm feeling out what actually makes a good group challenge.
Highest Tower of x width
Most Fish!
Largest FIsh! (they have sizes in the RAWs after all)
X-With-One-Blow - the ballista challenge. How many with one shot?
P.E.O. - PISS EVERYBODY OFF (how many SIEGES/WAR statuses can you trigger?)
FLEA CIRCUIS - most trained insects. (or perhaps most ADOPTED vermin? That's hard)
ROACH MOTEL - most captured roaches. In a cage. Just roaches.
WILLARD - most captured rats. in a cage. Capture a dwarf. name ''willard'' Lock ''willard'' in with the rats (transfer to his cage). Afterward - dump lot into the magma vent. Good riddance.
LARGEST POWER OUTPUT - what it says.
LARGEST POWER CONSUMPTION - again, what it says, but enough power must be available for it to actually run.
I think if multi-part challenges could be arranged, this would help, but I cančt think of many good ways to do this.