As a witness, I know that many are interested in the way that the top 20 witnesses keep their positions. It was also of interest to monitor the dead/inactive witnesses as it was speculated that some were deliberately voting for these dead witnesses to keep them in the top 50 to minimise the amount of real witnesses that steem users were seeing when they visited the witness voting page in steemit.com (which only displays the top 50). Currently though, this issue appears to have been resolved.
Identifying which witnesses receive votes from the same voters might be interesting in some way that isn't obvious to me at the moment.
Also, users who have received large delegations from Steemit Inc. should be a high priority, since it was claimed that the delegations were intended to help grow Steem but in some cases it is clear that the money has just been funnelled to the recipient and their friends.