There are three elements that can be used for titles: <title>, <subtitle>, and <titleabbrev>.
The first two are obvious. The third is typically used to designate a shortened title, which some styles use for notes. (e.g., CMOS short notes).
I propose requiring at least the <title> element and allowing the other two elements with the following interpretation:
If <title> is the only element, then any place that needs the title, including places that could use a shortened title, uses the value of <title>.
If <title> and one or more <subtitle> elements exist, but there is no <titleabbrev> element, use <title> for the shortened title and combine <title> and the <subtitle> tag(s) (with a style-specific separator) for the full title.
If <titleabbrev> exists (with or without a subtitle) use it for a shortened title and handle the full title as described above.
By allowing all three elements, writers can handle two edge cases:
Two (or even more) subtitles. For example, DocBook 5: The Definitive Guide: The Official Documentation for DocBook. In this case, the author can use <titleabbrev> to identify the desired shortened title (just DocBook 5 or DocBook 5: The Definitive Guide).
A very long title or a title that the author wants to shorten in a non-obvious manner. Again, the author can use <titleabbrev> to identify the desired shortened title. For example, Dr. Jekyll and Mr. Hyde as a shortened version of The Strange Case of Dr. Jekyll and Mr. Hyde.
Allowing multiple <subtitle> elements, which the DocBook schema currently does, gives the style flexibility in how to separate the titles. For example, CMOS separates the title from the first subtitle with a colon and the first subtitle from the second with a semicolon.
<title role="translation"> should be allowed in addition to the title to provide a translated title (e.g., in CMOS style, L'Étranger [The Stranger]).
Because the type of a <biblioentry> or a <biblioset> is defined in the parent element, I don’t think it’s necessary to add pubwork to <title> or to use <citetitle> in place of <title>.
The default and the ISO 690 stylesheets use <abbrev> to generate an inline reference and as the label for an entry in a bibliography.
However, it might be better to generate the text in these two cases from other information, such as the author's name and the pubdate, since inline references may or may not contain punctuation. I suggest it be allowed as an override but not required.