When you think about it, the weapons or armor transforming into a legendary items does make sense when you put materia into the equation. For the most part it'll most likley be gear that has been augmented into materia that will allow one to get said legendary weapons. Materia is the product of ones attatchment to an items they built up and then converted into another form so the lore idea that the materia's essence in the gear allows it to transform into a legendary items is very plausible.

Only thing that bothers me about this idea is will it be heavily luck based, I'd hate to think that Legendary weapons turn into a whoever has the most gil/luck item. When say you need 3 IV STR to get a Bravura Great Axe. I hope some skill can be put into this system it would really be a pity if it came down to people with legendaries either buy gil or have a lot of gil lol.
But ya, it would be cool if he can dive into that in the upcoming live interview coming up I would like to see that concept in more detail cause it defintely does have promise.