If anyone wants a good starter task, the editors could really use The editors' hall of mirrors and trickery but it isn't acceptably useful right now. It doesn't update the moved reply's "thread". If moved to be a reply to a root node, it doesn't update the number of replies on that root node (and likewise doesn't decrement the number of replies for the previous parent). Of course, it doesn't move entire threads. It also can't be used to turn a root node into a reply.
For an even smaller starter task, the "SOPWify" link for Cat Q+A (which is just ?op=sopwify&node_id=... mostly works but leaves the count of replies to the newly-created root node at 0. It also has a race condition of allowing a previously-started reply to get created as a Cat A that is a reply to a SoPW, though I'm sure that is much harder to fix.