This was brought about by this Reddit thread, where my response was probably not very helpful to the original poster, but it did get me thinking about the make potion spell.
Looking at the Spell Text
The caster creates mystical brews that grant supernormal powers to those who consume them. The result of the spell check determines which kind of potion can be created, as indicated below; each casting allows the caster to choose one potion from the eligible results at his spell check or less. This portion of the spell requires 1d6+1 hours to cast.
This suggests to me that initial spell check determines what potions the caster might know at the moment how to create from the list. If the initial spell check is too low, the caster cannot even attempt to make a specific potion. Perhaps the stars are not right, or the herbs required are not in season. Whatever the reason, without pondering and study (and a better spell check!) the caster can only do so much.
The spell as written lists what the caster can attempt to create. By using the Quest For It mantra, it is possible to increase the potions a caster might know based on spell check. For instance, the caster might discover a particular potion, and the judge then determines which spell check ranking it falls into. Once the spell check ranking is determined, the potion becomes possible to make for that caster.
The judge may also wish the caster to be able to research/experiment to create new potions. The same sort of rule would apply: (1) the player determines the potion's likely effects, (2) the judge determines where that would fall on the spell check results, (3) the judge determines what special ingredients might be required, (4) the PC has to obtain those ingredients, and (5) the PC makes a spell check. If it is high enough, the PC has successfully learned the formula for a new potion, and can add it to his list. The PC still does not have a sample of it, however.
Once a potion is decided upon, the caster must spend money equal to half the potion’s spell check number (rounded down) × 25 gp to procure the necessary equipment and base ingredients for the potion. In addition, each brew requires a special substance that must be harvested by the caster himself and then brewed, which takes roughly one week after the spell is cast. See below for suggested special ingredients and more details on potion effects. Unlike other spells, the judge, not the caster, makes the spell check roll to determine the caster’s success.
This suggests a second spell check, where the caster does not know whether or not the potion creation is successful until tested.
Options and Ideas
In addition to having recipes for potions being secrets that have to be learned/unlocked beyond what is in the make potion spell, you may also wish to read the section on ritual magic, short as it is, and then apply the ideas therein to certain potions. Yes, you can make potion X with ingredients A, B, and C, but it gives you a bonus to the spell check to use ingredient Z.
The Tales From the Smoking Wyrm zine has included some interesting rules for herbalism in issues #1 and #2. The Hubris book has an Alchemist class, which might have some ideas that you could use, even if you do not use the class itself. Several new potions are described in Danger in the Deep! as wares potentially sold by the snailtaur potion masters.
The arcane affinity spell could be used to unlock a new series of potions, if a caster gained an affinity as an alchemist.
Remember that you can also tie potions into seasons, astrology, or whatever you want. It is entirely possible to have a potion that can only be brewed during the first summer solstice of a new century.
The original poster in the Reddit thread was concerned about forgetting the new weird and powerful ingredients required for making a potion. The best way to avoid this is to take notes! You would lose a lot of the color of they system if you ignore potion ingredients. There is a big difference between having to harvest mummy dust and spending a few hours to make a generic spell check. The first is not only more flavorful than the second, but it is also more dangerous!
Spells as Potions
Obviously, the manifestation of a particular spell might have it play out a potion. I had one other thought, though: have you considered creating a Mercurial Magic table that allows spells to be learned and cast as potions? Some effects might just create potions. Others might give the caster a choice to use the spell normally or concoct it into a brew?
For instance, fireball could be "cast" in potion form, creating a liquid that explodes on contact with air. Depending upon the mercurial effect, the strength of the potion could be either known at the time of casting, or have to be determined when it is used. (Fireball potion +5 would indicate a roll on the fireball spell table with a +5 bonus, whereas fireball potion 25 would indicate the result on that table.)
Mercurial effect could also determine whether a potion imbiber or the caster suffered the effect of a natural "1"!
If you are even crueler, imagine the ability to create a potion that forces the imbiber to suffer spellburn to fuel the potion's spell check!
Just a thought.
If there is enough interest in this idea, I would certainly develop it further.