Future features from 3.0

[kahaaga]

This is a PR that spins off the V3 branch just before the V3 branch is finished.

Its sole purpose is for archival, so we don't accidentally delete the branch thinking it's unimportant. This branch contains a bunch of almost-ready implementations that I don't want to accidentally disappear, but that I wanted gone from the codebase for V3. Once #372 is released, the remaining functionality here that didn't go into V3 can go into separate branches/PRs. I'll get to that when I have time...

There's no need to comment or review on this.