Do we even know who build the ancient weapons (i suppose that ancient civilisation which was annihilated by the WG ancestors,… or the earth to counter Sephiroth, lol)
and do we know if they were ever used (likely, because otherwise no one would frighten/adore them so much)?
Well, considering that the leads to the Ancient Weapons are imprinted on the Poneglyphs which were created by that ancient folk, it's pretty much a given that they had something to do with it, but why were they annihilated in the first place when they had those weapons? is it because they abandoned that power in order to protect the world from total destruction or is it that they couldn't use them (maybe alien technology) and therefore hid them instead of giving the WG ancestors a chance to grasp that power?
...if i was them and couldn't use the weapon for whatever reason and i also wouldn't want that power to fall into the hands of the enemy, then i would have destroyed those weapons instead of hiding them... so can it be that those weapons really are already destroyed or do you think these people bet everything on the slim chance that someone else than the most powerful and influencial organisation in the world would find these weapons and overthrow the WG... which pretty much does seem like the same situation in which ancient civilisation vs. WG ancestors were themselves.
of course there is also the possibility that those weapons couldn't be destroyed.
the whole existence of the ancient weapons raises more questions than answers and i still fear that we won't be seeing much of them (not much action) even if it is a shame. the schedule is just too tight, those 10 years, i hope Oda has to go overtime.