• What – if any – existing elements are shown at the top of talk pages, immediately beneath the page title, that any design would need to consider?

A lot. Article history boxes ({{Article history}}), WikiProject assessment boxes ({{WikiProject Biography}}), archival-related boxes ({{Auto archiving notice}}, {{Archives}}, {{Archive}}), navigation & instruction boxes ({{Village pump page header}}), full Wikidata property documentations ({{Property documentation}}), just to mention a few. Sometimes folks even try to use unboxed descriptions to work around banner blindness, but I don’t think they’re successful, those tiny bits of text are simply lost in the shadow of the huge boxes. :)

I'm not sure how I ended up subscribed to this haha, but since I'm here, some thoughts:

• Regarding existing elements, if I understand this correctly, the thing that immediately comes to mind (at least for en-WP) is the talk header template. It has about 600,000 transclusions, but that includes pretty much every article talk page that could be considered active. I've thought before about whether it should be just integrated into the interface, so it might be a good starting point for your designs here. It currently still has elements telling people to place new sections at the bottom and to sign their comments, but those will presumably be removed once the software handles those things.
• Regarding the distinction in types of talk pages, you're correct about user vs. other, but I'd suggest a third, which appears in the project namespace: improving the associated page vs. improving the entire content area. For instance, at Wikipedia talk:WikiProject Higher education, the discussions are not just about improving the associated Wikipedia:WikiProject Higher education the same way that discussions at Talk:United States are only about improving the page United States. Rather, they're about improving all higher education articles. This is denoted by using "wp=yes" in {{Article header}}, which changes the bolded text to include "and anything related to its purposes and tasks".

Loose thoughts
In the coming weeks, we will begin sketching some designs for what the talk page frames could look like. Below are some ideas we will consider exploring as part of these sketches...

• "How might the page frame help people decide whether there are discussions they ought to consider reviewing before editing the article/template/file/tc. page to which the talk page is attached?"
  • Information: number of active discussions, number of discussions that have been archived since the content page was created, last time someone edited/contributed to the talk page, etc.
  • Actions: filter discussions present on page they are currently viewing, search for discussions related to a questions they have

The number of discussions in the archives seems a little useful, although we currently have a pretty good idea just from the number of archive pages. The number of active discussions is already clear just from the table of contents/length of the page. The last time someone contributed might help make it clearer when a page is inactive/unwatched, but there might be better ways to accomplish that.

For actions, one I might use if it became available is filtering to discussions where I've made a comment.

In T269963#7642550, @Sdkb wrote:

The number of discussions in the archives seems a little useful, although we currently have a pretty good idea just from the number of archive pages. The number of active discussions is already clear just from the table of contents/length of the page. The last time someone contributed might help make it clearer when a page is inactive/unwatched, but there might be better ways to accomplish that.

For actions, one I might use if it became available is filtering to discussions where I've made a comment.

This feedback is helpful – I'm glad you decided to share it with us, @Sdkb.

For actions, one I might use if it became available is filtering to discussions where I've made a comment.

• Can you share why you think you would value the capability you are describing? One idea: maybe you're thinking "discussions I've commented in" is a good proxy for discussions I am particularly interested in seeing?

• Do you have a sense for where on-wiki would be a good place to invite others to share what information and actions they might value being more visible / accessible at the top of talk pages?

For the first one, it's normally because I'm looking for a discussion I've contributed to to check if there are updates (less frequent now that I can subscribe) or to link to it from another discussion. (Now that I think about it, having a link icon that would copy the wikilink to a discussion would be super useful.) I currently do that by ctrl+f and typing in my username, but a box would make that easier.

For the second one, w:WP:VPI would be good to host the discussion, as it's a broad venue suited to the early brainstorming stage. w:WT:Talk pages project, w:WT:TPG, w:Help talk:Talk pages, Template talk:Talk header, and w:WT:Teahouse would be good places to issue invites.