I've got an NPC alchemist (Order of the Green Cockerel), I keep track of them as if they were a PC.
They were experimenting on their first Longevity potion and came up with a story event. This is kind of perfect, as I designed him to be a plot hook for the PCs.
Any suggestions for what sort of story could I build around a Longevity potion experiment?
As the work in the lab was about to end, the Magi finds himself on the floor of his sanctum, very tired. A few days later, suddenly all women within a few miles radius find themselves pregnant, and the pregnancy develops fast, over a few weeks rather than months., and all children take after him.
That's certainly a side effect, but where does the story come in? Not saying it can't/won't lead to stories, but it isn't really a story by itself.
On the other hand, information about the setting could also be useful... covenant location and situation, I assume the alchemist is a magus, not just an alchemist. It also depends on what kind of stories you want to run- the magus vanishes into the magic realm on a corpus insulae that is a microcosm of his own body, leaving a doorway open behind him could be interesting, if you like adventures in the magic realm. His appearance shifts so they seem years younger and are being accused of diabolism in the surrounding community... or their penalty for the Gift temporarily doubles for men and reverses for women as he also temporarily acquires the curse of Venus. He becomes a corpus monstrosity (vampire? lycanthrope?) any of these effects may require an exotic ingredient or material to reverse that you cannot get through the redcap network....
The alchemist is indeed a Bonisagus, Covenant is located in Great Yarmouth (East coast of England).
In a group of three he's the only person committed to traditional Longevity potions. He started to brew one after suffering a minor aging crisis. The other two members of the covenant have discovered an alternative way of achieving longevity through binding themselves to a dragon (It's a bit like the Familiar bond but in reverse, they drink the dragon's blood, suffer dragon-related transformations and have a loyalty bond to it.)
I did consider the covenmates forcibly binding their one hold-out to the Dragon, but they don't have a good reason to betray his trust.
I was thinking of some of the alchemical reagents that he used draining into the nearby river and having some dangerous effects on the locals who live by that river.
I once tried to prepare for this eventuality, and came up with an idea that might be over the top:
Every egg laid within the current AotH will hatch a basilisk/cockatrice. (There is a large tree inside the AotH)
Don't know what experiment deserves this.
Edit:
I recall one other - slowly the AotH loses strength, 1 point per day until it dissipates at 0. While deflating it makes a "whistling sound" that attracts nearby Supernatural creatures.
Dr. Jekyll and Hyde comes to mind.
So it works but he has some kind of Lycanthtopy like problem.
Or maybe nothing happens for now, but when he dies he becomes a vampire.
If you're thinking about having alchemical byproducts escape into the waterway, why have them have dangerous effects? It's a longevity ritual, so have there be beneficial side effects. All the old folks nearby are suddenly spry and vigorous. All the old animals are rejuvinated. It starts attracting a lot of attention: the Church investigating the "miracle," the aged seeking rejuvination, rival alchemists who're sure there's been a Breakthrough. It shouldn't even need the threat of Quaesitors; there'll be enough Story in this without dealing with them too.
And, heck, maybe it is a potential Breakthrough and the other magi might reconsider their dragon-longevity scheme.
There is some kind of area wide effect that the dragon's eggs hatch, and something that seems to be humans emerge (possibly fully grown).
The dragons are not happy about it.
Longevity potions are delicate and personalised things, so I don't think it would be 100% positive, if nothing else it would cause warping, but yeah it doesn't need to be negative either.
Having children born with blue skin or with magical blood. Or clothes washed in the water being magically animated.
Interesting! Not limited to longevity potion, but we have actually recently in my own long-running campaign had quite a bit of a side-story arc around original alchemy research and reconciliation with Muto and Vim - inspired by, but not really following 5e canon or TMRE (we tend to go off-script a bit so players can't just expect what's in the books, and it's more fun).
In (very) short it has been both a political and research challenge across a dozen or so sessions and a tribunal, where the player side has advocated Magic and Alchemy reconciliation - while hardcore old-fart Bonisagi (and Tremere) have been more or less directly hostile against it (in an academic and political manner only).
I probably have 50+ pages of material, including several session chronicles, handouts, passive-agressive letters, tribunal-debates and a ton of detailed alchemy+magic original research ideas.
Unfortunately it's all in Swedish, but I've considered some translations to share.
Let me know if it sounds like something you are after. It could be that the experimentation on longevity potion opened the door to new discoveries and the magic + alchemy true reconciliation.
The crux of the matter in our campaign was really the limitations of Muto and true matter, vs Alchemy limitations of Tria Prima. Only way past either is to combine them (and the whole thing kinda exploded).
To give you another idea (that basically happened for us in a separate arc) is fathering of a special child despite the fact that the magus should be infertile. It had all sorts of connotations - including quite a few political and research related. The big one was huge omens at the birth, prophecies from several diviners (including the fae), and even arrival of inspired pilgrims to witness the birth. The arc is a long time from concluded so I can't really share it here, but the kid is growing and it may be major 
One may also wonder why the heck take the risk to experiment with your longevity? (also, I'd consider that faerie or infernal vis was accidentally taken for regular vis and used - with some significant consequences).
This thread partly inspired me to finally go and refresh my campaign chronicle translation including the arc, handouts, letters and tribunal debate mentioned above.
I've updated my campaign post with the details and link if anyone should be interested.
You can find the alchemy arc near the end (contained within session 89-97 - just search "alchemy", and originating with two handout letters (Ricardus to Guiscard) that are before #79 and #89 (the players were a bit slow on the uptake on this arc, busy with all the other craziness).
Here's a direct link to the first letter that kinda kicks it off:
https://docs.google.com/document/d/10td_pRNap4pv72N06DfB-zCaDwbJ9UIayzjkMqxZI0U/edit#heading=h.4e4bwxm
FWIW: The Handout between session #96 and #97 contain no less than 14 avenues of potential magic and alchemy reconciliation discoveries and breakthroughs that my players (or other magi) may try to pursue if you wanted some ideas on that. To tie it back to the thread, this is what we have intended as more or less potential story events by themselves.