Here's my use case: a client will be making numerous blog posts, some of which are portfolio pieces of his recent work. This sounds like a perfect use case for a Post Format, but no "portfolio" post format exists.
Post Formats are meant to be standardized, common formats, such as gallery, quote, status etc. Unfortunately "portfolio" isn't in that list and doesn't seem to fit with any of the sanctioned post formats.
Creating a custom post type for this seems like bringing a bomb to a bonfire and frankly doesn't sit right with me since the portfolio blog post fits right in with the chronological main blog.
So should I consider extending post-formats (if that's even possible)? Or is that strictly taboo? What's another alternative?