Skills:
20xp Alchemical Theory - buy in skill, required for all other skills on this page. Grants the ability to call RECOGNISE potion.
10xp Alchemical Ability N - grants 3 (or 4? or more?) alchemy points. May be taken multiple times.
5 * [Level] xp Make [type] potion N - grants the ability to make a level N [type] potion; requires the prerequisites listed under potion trees at level N-1. When double-buying only pay double for the lower of the two, possibly (though this encourages powering through to the top potion in any tree too quickly).
The structure this aims to encourage is that at a low level all alchemists are pretty much alike - everyone can make a very basic poison or potion - there should be enough trees to fill an entire level even at level 1 - but then they specialise at a higher level (or, alternatively, you could have a level 8 alchemist who knows every potion up to level 3 or 4 - very versatile but not so powerul); every effect in the system is available to an alchemist, but of course they have to decide which they can make and then prepare them beforehand. XP cost should be such that the top level of trees with other prerequesites is available only to pure alchemists unless you concentrate entirely on that and ignore all other potions, wheras something like 8th level healing is available to a reasonably even and unmunched multiclasser, or someone who double-buys ability to be able to make lots of potions - the costs I've given are probably too much, since they work out something like:
Level 1: Buy-in, ability, 6 trees 60
2: ability, 5 trees 120
3: ability, 3 trees 175
4: ability, 2 trees 225
5: ability, 2 trees 285
6: ability, 2 trees 355
7: ability, 2 trees 435
8: 1 tree 475
The buy-in in place of class skill should discourage people from taking a level of alchemy to just get one or two potions, and could possibly be make even higher, e.g. increase it to 30xp and make the make potion skill 4xp/level, then it goes
1: Buy-in, ability, 6 trees 64
2: Ability, 6 trees 122
3: Ability, 4 trees 180
4: Ability, 3 trees 238
5: Ability, 3 trees 308
6: Ability, 2 trees 366
7: Ability, 2 trees 432
8: Ability, 1 tree 474
I think this is the dynamic we want - if you're an alchemist, you tend to be a good one, there aren't many dabblers - though such a high buyin would mean those who start with alchemy are pretty much stuck as alchemists and very few people would become alchemists in play.
The system part is independent; I needed one to play around with so made one up. I haven't yet done anything about the "alchemists do nothing in uptime" problem (though the trees are mostly written with the assumption that the alchemist will be the one applying the potion), but any solution to that could hopefully be bolted on to this kind of xp structure, which is the main thing I'm trying to get at:
System: You have some alchemy points, which regenerate each week. Making a level n potion costs n points (optional: more if it is from a tree with prerequisites? Per-tree prices are also vaguely feasible given that buying the potion is done in downtime) and, I don't know, 3n shillings, and probably some herbs, and requires the skill make [type] potion n (numbers can be fiddled with); maintaining a level n potion costs n/4 alchemy points if you posess the skill make [type] potion n, n if you do not, but no money; unmaintained potions become unusable after a week.
Likewise what trees are available isn't really what I'm trying to concentrate on here - this is more a "front end" issue and is being dealt with by e.g. Jacob/WhatAlchemyCanDo. I've put some here just for the sake of something to work with, but it's the XP structure I want to get right rather than the specific potions.
Trees:
(some of these are overly similar to miracles/spells - I lack inspiration. I'm more concerned with the general idea - these probably aren't balanced, they're just their to give an idea of the sort of thing that should be available)
Healing potion - requires healing potion. A level n healing potion cures 1/3(n^2+1) (rounding up) hits when drunk, i.e. 1, 2, 4, 6, 9, 13, 17, 22. Possibly. Or maybe 1/2 (n^2+1) so 1,3,5,9,13,19,25,33. Something like that, anyway - it needs to be better than linear, otherwise there is no point in the higher level ones. Maybe work in some special ones like regrowing a severed limb over the course of a week at the high levels.
Ingestive venom - requires ingestive venom - hits probably hand-tuned since instakills really suck, something like 1,2,3,4,6,8,12,lots
Blade venom - requires ingestive venom and blade venom - as IV but it's a blade venom.
(Possible idea: arm venom for disabling combatants, does same hits as IV but spread evenly (as much as possible) between the two arms rather than to body. Blade venom could depend on this too, if it's currently too easy)
Incapacitating potion - requires incapacitating potion - something like silence 500, halt 500, weakness 500, blindness 500, inflict silence, inflict weakness, inflict blindness, inflict freeze.
Incapacitating blade venom - requires incapacitating potion and something else, perhaps immunity potion, not blade venom because that makes it too easy to be a pure combat alchemist.
Immunity potion - requires immunity potion - can be used either to inflict: you may call negate to 3 calls of [call], or to negate one call of [call] when in progress, where call is silence, halt, strikedown, repel, blind, freeze, I dunno, inflict or something.
Docility potion - require docility potion, behaviour alteration (or else that requires this) - inflict: you may not attack someone not bearing a weapon or a known caster, someone not holding a weapon or casting, you may not draw a weapon on someone not holding one, you may not attack, you may not even defend yourself, this kind of thing - would be better with a timed duration, but I don't know if there's a way to do freeform effects with timed durations in system
Behaviour alteration - require behaviour alteration - stuff like feel happy, irritable, invulnerable, love potion, love potion, beserk, take -1 degree damage.
[More trees]