Hey Pierre,
Could we have an additional command in the Item | Subitems... submenu to toggle the collapsed / expanded state of an item? That way, a user could always use the same shortcut, e.g. Ctrl-PgDn or Ctrl and either of the horizontal arrow keys, rather than having to make up his mind whether he needs Ctrl-Left or Ctrl-Right.
Generally, for special requests like this, I guess it would be useful to actually not have these commands show up on any menus by default but only to make them available in the keyboard shortcut configuration dialog. This would allow under-the-radar power user functionality without cluttering up the general interface for people not interested in such additional functionality or alternative UI options.
Cheers