I actually don't like #region in my code. BUT for some reason call me crazy, I would like to have them in my XAML. I would like whole sections to have a #region-like thing and collapse them (e.g. my <Window.CommandBindings>, <Grid.*Definitions>, <Menu>, <Toolbar>, etc..
Does this exist? If not, how about <RegionCollapse>
No, but according to this reply by Microsoft:
This is a great suggestion. Thanks very much. We will consider it for a future release
I might be missing the point of your request, but the XAML editor automatically puts collapsible regions around the elements. Note the nodes in the left rail of the editor window. Click the "-" and the associated element collapses to a single line, much like a #region.