To -1 or -2 your own code is a decent workaround, but that doesn't
communicate much about the actual status.

When voting, just use the comment field to describe in greater detail
why you voted -1 or -2 on your own change. That should serve the
purpose just fine.

The "WIP" prefix in the commit message's first line is not special to
gerrit, so you can replace it with what ever seems appropriate to
you. Although "fixme" sounds confusing to me, you can of course prefix
by "fixme", if you choose to.

I guess, if we were able
to put a patch back into the "draft" state, [...]

I doubt that going back to draft state would get love from
upstream. Generally, upstream does not like the current state of
drafts too much, as they raise complexity a lot. Going back and forth
between draft states, would raise the complexity way further.

Also, drafts come with different visibility. Allowing to go back and
forth between draft state strikes me as confusing for users:
“Hey where is that draft gone? It was there a minute ago”

I would like it to
disappear from the main watchlist, [...]

If you intend to fix it soon, the “WIP” prefix typically works fine.

If you do not intend to fix it, just abandon the change. That makes
the change disappear from most of the change lists. Star the change,
so you can find it easier afterwards.

[...] it could not be merged, and

Vote CR-2. That hides the submit button for people.

I'm guessing this is a missing feature in the product.

This is now supported in gerrit 2.14 (not released yet) I think but only admin's can convert open patches back to draft.

Actually there's a plugin https://gerrit.googlesource.com/plugins/force-draft/

@demon could we install plugin https://gerrit.googlesource.com/plugins/force-draft/

No.

I have created this https://gerrit-review.googlesource.com/#/c/97062/ to allow us to delete changes, so really you could do it from scratch if we allow users to delete there own changes.

@demon I'm proposing this task as declined as upstream are discontinuing the support for drafts and replacing it with private edits, which gerrit will migrate existing patches from drafts to edits.