Look Out It's A Plague™

A little while in the future (say this spring, no later) a new type of disease pops up. At first, only a few cats are affected mainly from Thunderclan and Riverclan. The medicine cats from both clans give the sick cats what they can and send them on their way. Unfortunately, the herbs don't work, and symptoms begin to show. Symptoms include a burning sensation in one's stomach, intense headaches, lack of balance, and in the later stages, foaming mouths and even insanity.
Hearing of this 'incurable' disease, Windclan decides to essentially put their entire clan under lockdown as a precaution, and no strangers are allowed near the borders. Skyclan remains neutral and Shadowclan cats are now traveling between Thunderclan and Riverclan, distributing herbs and food. All sick cats are kept under quarantine away from all the territories, in the Barn. A few cats from Skyclan and Shadowclan step forward and offer to care for the sick cats at the barn.
For a while, everything seems to be at a standstill. But somehow, this new disease is carried into Windclan by a delirious sick cat who is attacked by a foolish apprentice. Skyclan calls an emergency meeting at Fourtrees as the disease spreads faster, and the few healthy cats attend and somehow come to the conclusion that the disease can only be cured by a rare herb only found in Skyclan territory.

Skyclan holds a clan meeting and invites the other clan's medicine cats. There, they decide to either share the herbs (with either all or a few of the clans)or keep the herbs. Depending on what the choice is, a dangerous battle breaks out, or the disease is stopped once and for all, and new ties/alliances are made by all the clans. In an ideal situation, Thunderclan and Riverclan would be forced to put their differences aside and fight Skyclan for possession of the herbs, and Windclan and Shadowclan would join together in an attempt to find another cure. Eventually the herbs are won over, the conflict is resolved, and the medicine cats travel together to each clan, and eventually the barn, and within days every cat afflicted is completely back to, or close to, full health (long-lasting effects can take place, such as insanity later in life or random headaches. It's up to the rper).
Later herbs are planted throughout all the clans and the disease is named.

Thunderclan and Riverclan cats get sick with a new incurable disease. Windclan goes into lockdown, Skyclan remains neutral and Shadowclan desperately tries to help both clans as best they can. The disease takes over 1/3 to 1/2 of Thunderclan and Riverclan, forcing the other clans to create a quarantine for all sick cats at the Barn.
A sick cat wanders away from the barn and ends up in Windclan territory. Meanwhile, signs of the disease are beginning to show in Shadowclan. Skyclan holds an emergency meeting at Fourtrees and a few healthy cats attend. They somehow come to the conclusion (after a few contributions from the medicine cats and their expiraments on the diseased cats) that the only cure for the disease is in Skyclan territory.
Skyclan holds a clan meeting shortly after, and invites all the medicine cats. At this meeting, they decide either to share the herbs (with all the clans or a select few) or decide to withhold the cure in an attempt to gain more territory, as theirs is the smallest and they struggled during Leafbare.
At the end of the plot, the herbs are won over somehow. The medicine cats heal all the diseased cats, and most, if not all, return to full health only a few days later. Long-lasting effects are up to the roleplayer themselves.

Although it would be fun to have Thunderclan and Riverclan continue to fight each other/be on opposite sides, it would be very interesting to see two opposing sides be forced to join together to fight a common cause. It'll be an amazing way to explore the Thunderclan-Riverclan dynamic, and for individual cats to forge friendships that will later be tested during battle.
Another reason I like this plot is that it's so different from what we usually see on WCC. Most of the plots center around a battle, forbidden romance, or something involving murder/the Dark Forest. This plot focuses on an external enemy that can't be beaten with brute force. It'll force warriors to think instead of fight, and often times that's much more interesting to roleplay, and will allow characters who aren't strong in battle to shine and take the lead for a little while.