Specifically, it would be nice to be able to select whether you want them to be CC'd, or assigned to that diff.

We are somewhat unlikely to allow users to automatically become reviewers, because this went poorly at Facebook (revisions ended up with lots of automatic reviewers and everyone kind of assumed someone else would review them, so no one actually reviewed anything). Removing this capability improved things. After T1279 we might revisit this, since we could clearly mark automatic reviewers. I also may just restore this at some point to align with user expectation, even though I think it's a bad feature and we benefited on the balance from removing it.

Makes sense. The biggest thing we would like to do over here is basically a workflow of:

[New Diff] -> {"Oh, you are the listed expert (owner) on the package that this diff touches, so you should be the one to review the diff."} --> [Assign to $EXPERT]

Perhaps it would be nice to do something like have Arcanist suggest people to review the ticket when you make the diff (based on the top X results of the 'arc cover' algorithim perhaps), but what we're primarily worried about is people not knowing who to assign the diffs to when they make them. What i fear this will lead to is differs randomly asking someone in whatever channel (we use jabber) to do their diff, which is not the best way to do code review IMHO.

See T1259: Improve "arc cover".

I also may just restore this at some point to align with user expectation, even though I think it's a bad feature and we benefited on the balance from removing it.

(We did eventually restore this because it was easier to let users do it than argue that they shouldn't.)