If EVERY episode had commentary I'd pick up EVERY season that a Star Trek series has ever produced.
This is actually one of the simplest special features to make and low cost if you ask the people to do it and not pay them.
You are talking about using the Narration booth and hiring a dialogue editor to edit it and master it.
I'm glad more and more TV shows are adding commentaries. "Mad Men"
has commentaries on every episode. Multiple commentaries in some cases! Actor commentaries are usually not great but if that's what it takes to fill up every episode getting a commentary...
How about letting a different department do a commentary? Art Dept.?, Wardrobe Dept.? Sound Dept.? Music Composer & Sound FX? Writers?