I have been attempting to implement the following use case:

I have a gnus group that receives messages with subjects that look sort
of like this:

Some title, part 1, by X
Some title, part 2, by X
Some other title, part 32, by Y

I gather these into threads by title using my own simplification
function which I add to `gnus-simplify-subject-functions' in this
group.  I also set `gnus-thread-hide-subtree' to `t' in this group, so
each title takes up one Summary line.

I then want to sort these threads by the *latest* message in the
thread.  Thus, when a new part is received, the entire thread shows up
as "newer".  To do this, I created by own function to set
`gnus-thread-sort-functions' to.  This looks like this:

(defun md5i-thread-sort-by-most-recent-date-reverse (h1 h2)
  (<= (gnus-thread-latest-date h1) (gnus-thread-latest-date h2)))

Unfortunately, this fails in many cases due to the fact that
`gnus-summary-prepare' gathers thread after sorting threads.  In my case
I need the reverse, and I think that the reverse always makes sense.  I
have a patch which does this, and has been working for me for half a
year or so.  I suggest you add this or an equivalent change to Gnus.
Feel free to rewrite this patch completely.  I wrote it long enough ago
that I do not remember why I needed to create `gnus-make-threaded-sort'.