I have recently created my own personal challenge that as I have mentioned to several other cachers they found interesting except they were not willing to do the work required to track the stat themselves.
Project GC Has all this data - just a matter of presenting it and creating some search functionality for it, likely using existing database functionality.
The goal would be to find caches placed on as many unique dates as possible. So January 1, 2019 would be unique from January 2, 2019 as well as unique from January 1, 2018 and January 1, 2001.
I currently have finds on something like 3540 unique hide dates out of a possible 6846 total findable days since 5/11/2000 when Mingo was hidden. By my calculation there are 171 days since then with no active caches and I dont count those, (but some will have found caches placed on days with now archived caches...)
Seems like a good scoreboard type stat - how many unique placed dates you have and your ranking against cachers for that stat.
This would also add all kinds of unique placed date type challenge caches for people to work towards - find caches placed on XXX unique days, find YYY caches hidden on unique days before 2010, find ZZZ consecutive placed days (my highest is currently at 32 - and I have two streaks that long - better believe the two unique dates at the beginning and end of those streaks have been identified and targeted). Find 10 caches that are the only remaining cache for that unique hidden date... etc etc
Once you start tracking the data this would make for a useful search functionality as you plot trips or look for nearby caches to work towards a goal of extending a streak or placed dates that may not be near your home. Not all unlike existing searches for D/T, Jasmer, and calendar dates.