I suggest alchemist to do these sort of things:
- turning one type of material into another. For example, turning 20 lead bars into 1 gold bar (the amount of bars needed may depend on how skilled alchemist is)
- brewing some potions, that would buff dwarfs, like making warriors more strong and tough or speeding up healing, but long usage of them may result in dwarfs getting addicted to them.
- creating poisons to make dwarven weapons poisonous
The last two would be good, but the first one is horrible. Poisons are already made though, but they are just simple extracts, I assume that alchemists could make far more potent and far ranging poisons, maybe through some system like The Elder Scrolls has, where you mix the ingredients and the effects are the intersection of the individual effects of the plants. The problem is that poisons need to be implemented properly first, i.e. you can apply them to weapons.
The idea of buff potions is pretty much the same as above, so no more elaboration is needed.
The idea of turning lead into gold is a really bad one, as it is completely non-realistic and gamey. Yes, it is possible, but it hasn't been done yet, the only true alchemy we have done is atomic fusion and fission, which aren't even remotely feasible in DF. We have managed to convert platinum into gold in real life though, but all that required was jamming another proton onto a Pt atom, which is very hard to do in itself, let alone pulling off half a dozen protons. Also, Pt is worth more than Au.
One use of alchemy that is often suggested is that it could be used to create gunpowder, except then there's the whole discussion on whether we should have gunpowder or not.