I've always wanted to see something like FFT's Chemist as a healer, where you get to lob potions at people with an "Item" command. Bravely Default's Salve-Maker was similarly interesting. Given that Thavnaria seems to be a hub for alchemy, you could attach such a job to their lore.

You don't necessarily need ammo to do it (especially since "ammo" jobs don't really have a max ammo capacity or require purchasable ammo). You could have all the heals be built around cooldowns (i.e. you can throw a certain number of potions before having to refill/"reload" your flask) or around some base material/ether that you construct your potions from that generates over time.

As far as mixing potions go, there are two ways that you could do it. The first is that Mudra concept where you pick a combination of ingredients to get a particular result. The other, more organic option would be to use ground effects. Lob an X-potion at the tank. While they're sitting in a pile of healing goo, watching their health tick back up, you can toss a second potion at that puddle which combines with the first to create a new effect. You could just as easily do poisonous effects that harm opponents in similar ways. You could even introduce in something like FFXII's Nihopalaoa to produce a reversal effect on enemies.