Always found it odd. The Zodiac weapons were 12 weapons that belonged to ancient heroes, but as is we only have 10(if you count Summoner and Scholar as two different ones. They are sort of like the Gemini of the group.). I can honestly see the Dark Knight and Astrologian having one each, and I am sure Machinist could easily make one on par with the zodiac weapons(kind of like the 13th zodiac symbol).

There's also something we have to remember: The zodiac weapons we have now are COPIES of the real deal. They look similar, but they aren't the genuine articles. What if the expansion allows you to put together the real Zodiac weapons together and perhaps use your copy as the core? So your stats would already be customized for you, and would possibly be even bigger and not require you to do some of the more grindy steps?