Collect aggregate stats around hidden service descriptor publishes and fetches
How many hidden services are there, total? Is that number going up or down? Are there interesting recent trends? Does ahmia's list of hidden services represent most of them or few of them?
We have no idea.
If each relay that serves as an HSDir fetch track of number of .onion addresses they've seen, total number of descriptor publishes they've seen, and total number of descriptor fetches they've seen, we'd get closer to answering some of these questions.
There is also a privacy question here: a given HSDir is responsible for a given slice of the keyspace, so if indeed hidden services vary widely in popularity, then I expect the answers here to vary widely by relay. How bad is it to reveal stats that might be used to guess about popularity of hidden services? If it is bad, this might be a perfect situation for using a tool like PrivEx, to get the network-wide total without revealing per-relay contribution to that total.