Well, a possible plot idea for 'a bittersweet potion fanfic' is that the potion is created by an inexperienced wizard. The wizard doesn't realize that the potion has the power to make people experience their greatest fears and joys simultaneously. As the story unfolds, different characters drink the potion accidentally, and we see how they deal with these intense and conflicting emotions. It could also be that the potion was meant for a noble purpose, like curing a plague, but has some bittersweet consequences like changing the appearance of those who take it.
In a 'a bittersweet potion fanfic', the potion might be a forbidden elixir. It could be that in a magical academy, a student discovers the recipe for this potion in an old, dusty tome. The potion has the ability to give the drinker the power to control time for a short period, but it also makes them age a little bit each time they use it. This creates a moral dilemma for the character. Do they use it to correct past mistakes or to gain an advantage? And as they use it more and more, they start to lose their youth, which is the bittersweet part of the story.