Okay, another running idea, before I get to work on my other crap. This works best in a coastal, fair sized city, which is surrounded by a few marshes. It will probably require a fair bit of pre-work on your part.
Benign entry point: As the party is on their way to buy or sell stuff, or perhaps to the tavern, or to their inn of choice, describe a moderate hubbub going on in front of a large, four story town hall sort of building. There are four lines of rather poor people being interviewed by what are clearly four mages and about twenty private guards. Have the lines extend out into the street so that they block the way of the party, and try to build up the party’s curiosity in what is going on. When they ask, tell them that the “Mage’s Guild” is looking for test subjects.
Point two: Have the bell chime on the impressive 80 foot bell tower which is across the wide avenue from the Mage’s Guild and in front of the party. Have a nearby NPC remark, “Sure is a shame they’re tearin’ that building down. It’s a landmark, it is.” If the party doesn’t ask, have someone else say within earshot, “Why are they tearin’ it down?” -“Well, it’s a danger, ain’t it? They been sayin’ for months the ground’s been seepin’…ain’t stable, and that’s the truth.”
At that point, as if the Gods themselves were listening, have a deep sucking sound occur behind the party. As they turn, describe a part of the street falling into a sinkhole. Have the sinkhole widen rapidly, then describe a sewer-like rush as water down a pipe: the sinkhole heading at the party. The street people will scatter. Give the party a few doorways to leap into, and have the sinkhole pass right up the street and by them, and stop at the bell tower.
Describe some awful, disturbing plopping sounds, as though a titan was disgorging his dinner into the basement of the tower. Show the tower visibly quivering. Make it clear to the party that it looks as if it will fall. If the party say’s they’ll run, point out the sink hole is about thirty feet deep and appears soft at the bottom, and emphasize the question, “Which way do you intend to run?”
Now, while they are making up their mind, describe a booming belch emerging from deep, deep beneath the city. Describe a large bubble of air rising out of the muck at the base of the sinkhole under the tower, which then POPS!, splattering mud everywhere. And then have the tower fall over. Into the mage’s guild.
Begin the fun: Imagine, if you will, every conceivable potion, from the combined inventories of thirty or forty laboratories, suddenly being released into the atmosphere. You would do well to emphasize early on that the “test subjects” would indicate a great deal of research going on in this particular mage’s guild. What you want is to create a table which will ascribe the effects or reverse effects of the various potions you can contrive to add to that table, along with side-effects like insanity, gigantism, undeadism, etherealness, berserking, genetic modifications, mutations and so on.
Have everyone in the party affected, randomly. Have everyone on the street (and adjoining street, and additional quarter of the city, in the direction the wind is blowing) affected. Have the various animals, from large mammals down to minute insects, affected. Roll randomly for persons immediately adjacent to members of the party, including their reactions (which might be interesting if they turn paranoid or megalomaniac).
Sit back and watch fur fly.