Yeah, alchemists in FFX-2 had guns and the mix ability. Chemists in Final Fantasy Tactics were also one of the only classes that could use guns. While alchemist already exists as a crafting class, I think adding chemist as a battle class would allow for a healer that differentiates itself from scholar and white mage pretty dramatically in play-style.

There are a lot of different ways they could go with it, although I think the major rule would be that it could not actually require consumable items. They could do a charge-based ability system similar to scholar's aetherflow or warrior's defiance. It might also be interesting if they do a Mix action that is similar to the upcoming ninja's mudras. For example, as they level they could get access to different reagents that they could combine for different effects.

What I really want out of 3.0 is second jobs for each of the existing classes that occupy a different role from their current ones. I'd love to see gladiator get dark knight as a DPS alternative to paladin. Similarly, I'd love to see marauder get viking or berserker as a DPS alternative to warrior. Geomancer as a magic-based DPS for conjurer. Templar would make a great deal of sense as a tank for lancer, especially since the temple knights are part of Ishgardian lore already. Ranger as a pure DPS alternative to bard (which, as the level cap rises, should become increasingly focused on support).

Second jobs for puglist (dancer or puppetmaster?), rogue (...dancer?), and thaumaturge (...time mage? red mage?) become increasingly difficult to imagine, especially given the limitation of the holy trinity of healer, damage dealer, and tank. It becomes even more difficult if you want to make sure each second job has a different role from the first.

Speculation is too much fun.