Paraphrased request from @carl: Track who closed tasks in Maniphest, and the date they were closed, and surface those fields in ApplicationSearch. Motivation: Review progress made during weekly sprints.
Paraphrased request from @chad: Track when everything was closed, and expose "closed recently" as a global search filter. Motivation: in most searches, open and recently closed things are usually more interesting than just open things.
These are both fairly easy technically, although they tend to make already-complex interfaces more complex and the Maniphest flavor might bump into other things a bit (on the other hand, it might make them better/easier).