Matt, I don't see how to find and use the two new IPTC core fields, TITLE and HEADLINE. They aren't listed anywhere that I know of. Also, I don't see anything about > that is different.
Reminder, the new IPTC/XMP fields should all be single-value free-form. They should not be cross-connected or aliases to each other or to any other MC fields (TITLE shouldn't derive from NAME). They are simply individual stand-alone text fields stored as IPTC and XMP, and handled in MC as simply text fields, usable in views, thumbnail text, etc.
TITLE - Confusing because usage varies among apps (and among reference materials). Apparently no IPTC rule about using it but common practice over the years was a short human readable reference for the image. It can be a text reference or a numeric reference, and serves primarily as an identifier. TITLE in IIM format is officially limited to 64 characters.
The first pass of adding TITLE to MC26 connected it to NAME, but they aren't the same. NAME seems to be an internal-to-MC ID/key with no IPTC equivalent, whereas TITLE is an IPTC core standard strictly used by the photographer to efficiently ID the image, regardless of how/where it is stored (old film negatives/prints used this before there were databases, for instance, I have some that are decades old, 4x5 sheet film but the shots for a given day/job have the same TITLE, something like WPHS-196202...but some other photographer or photo shop might use it very differently. It's important that TITLE simply be a text field that MC stores but does not alter. .
However, Adobe and some others now seem to use Title as if it is the photo's headline. SmugMug displays it in the largest font above the photo. But really, that seems to be why IPTC provides field Headline.
HEADLINE -- A headline is a brief synopsis or summary of the contents of the photograph. This can be the fun part of a photo. Like a news story, the Headline should grab attention, and telegraph the content of the image to the audience. Headlines need to be succinct. Leave the supporting narrative for the Description field. IPTC says, "Do not, however, confuse the Headline term with the Title term." Easy to do, a catchy Headline is quite different from a dull ID number. BUT Adobe and SmugMug display Title as if it is the Headline (??). The field length in IIM format is 256 characters.
Because of inconsistent use by various apps, MC should provide both TITLE and HEADLINE fields and let the user choose what info goes where. (Adobe is a stone-wall, but I'll ask SmugMug's product manager (who claims tight Adobe integration) how they use and differentiate these fields.
DESCRIPTION/CAPTION - This field already exists in MC and works well. Note that the field is commonly called CAPTION (as in MC) though DESCRIPTION is the official IPTC\XMP field name. The IIM format field length is 2000 characters.
Re using > symbol instead of slash to indicate image Keywords nesting, I don't find how it is supported. It's always been possible to type in > to indicate hierarchy (Disney>Walt instead of Disney\Walt). The missing ingredient is having MC use > to trigger nested display in views (as it uses \). But nothing happening with > that I see.
(Is MC somehow withholding these fields and nesting unless it knows the library is devoted to Images? How would that be indicated beyond simply loading it with 70K photos?)