I've always liked the idea of an Alchemist being a combat class. Only MMO I've ever seen do it is EQOA and it was implemented in a really unique way. The class had a few spells, but its primary form of dps was throwing explosive potions that it brewed. But each explosive potion thrown took 1/8th of its total mana, and it had a couple spells that would trade health for mana restored, and then said health could be restored using a their own type of health potion. These potions of course were more like a summon or generation spell, as opposed to actually crafting them.

As you can imagine due to this design the class was focused on having as little maximum mana as possible, because the thrown potions took 1/8th no matter what the max mana was, and then the restorative potions would restore a lot more mana.

I think a design like this could fit really well into FF XIV being that we already have BLM and DRK who have to carefully manage their mana reserves during the fight, albeit in different ways.