Scythes would be tricky as they would probably require entirely new animations. On the other hand I could see them using the Axe animations for Hammers. I think the man issue is the issue with Slashing damage but technically they already have a metal club for DRKs so maybe that isn't as big a deal.

I think one of the reasons they limit weapon options is that most Jobs are identified by their weapons and no jobs share weapons. Even SCH and SMN use different books. It allows them to have clear variations for Job identities.