Arcanist (aka Arc) is the Phabricator tool to upload patches for code review. You can find a reasoning for Arcanist to exist by its creator. Is it missing anything essential for code review in Wikimedia?
There are many tasks related to Arcanist, and many more that might come. However, which ones are the real showstoppers that we either fix in Arcanist or find workarounds elsewhere?
We need to identify real showstoppers and their potential solutions in order to plan the code review migration accordingly (T18).