I’m stocking up on crafter materia and gatherer materia. These will sell incredibly well next xpac when crafters have to get new BiS gear, which will certainly use old materia for filler.
Also, extreme primal materials, because I expect at least some new craftable primal weapons (Shadowbringers was a massive letdown in that aspect, being the first xpac to not release every previous craftable primal weapon for the new jobs), which will sell for millions of gil on release.
Then I’ll precraft a bunch of leve items from the highest level levequests for crafters, to get a headstart and boost all my crafters to level 81 for the new leves. You can math out how many leves you need to turn in to jump that gap, based on previous exp tables.
Other than that I don’t know anything special I’ll stock up before the xpac.