git annex sync and the assistant do a merge of new revs, and then download the content of files. However, this means that broken links can show up, when a file has changed, or a new file was added. In some workflows, the user would prefer not to ever see such broken links (or at least never for files that are in the repo's preferred content).

So, how about a new mode, that defers updating the work tree until the content of everything wanted is available?

This could be a annex.merge=downloadfirst setting; it would make sync/assistant look at the diff between HEAD and the new rev, and try to get all annexed files added in that diff, before merging it.

Of course, it could take a long time to get to see a new work tree. Might have to download a lot of content.

What to do if it fails to download a file's content? Could either abort, leaving the current work tree as-is, or could go ahead and merge, letting broken links show up in this case. I kind of prefer the abort option. But, if the content never reaches any remote, or has gone missing entirely, that would make sync never succeed. That could be surprising behavior.

Probably best to let the user pick either behavior, so annex.merge=trydownloadfirst and annex.merge=reqdownloadfirst

Let git annex merge be used to force a merge, even when content is not available.

alternatively

What about just making git-annex sync --content try to get the content of all files before updating the work tree? (The assistant would need changes too; it would need to queue all the downloads and trigger a work tree update once all the downloads have been tried.)