I LOVE the magical plague idea. Plus Briony would love to contribute to finding a cure. In my mind I always assumed that although healers specialise in certain wards, when demand is too high they're transferred to where they're needed. So that could potentially involve healers worldwide working together to try and solve it. I also would be the first to sign characters up to get ill, because illness is super fun to play and would get a lot of takers.
On the striking-back-at-deatheaters/purebloods front, I like that too. This might just be the me that's studying the Treaty of Versailles right now speaking, but the way the allies that surrendered at the end of WWI were punished by everything else could be usable. I completely see there being complete mistrust in purebloods and their ideals, and people wanting revenge much more than justice in the form of an azkaban sentence. The ministry could be involved there too as they'd likely come under pressure from the public to punish the perpetrators asap and worry about the long-term later.
*backreads some more*
Victory ball. Me gusta. The disastrous side sounds good, too. Because if I were a magical creature I'd feel upset at being controlled and used as pawns for the past year in a wizarding battle too. Maybe the goblins could kick off also? The wizarding world is long overdue irate goblin drama! Bringing the centaurs into it would be a brilliant starting point for the people who wanted to make them anyway, I for one was avidly following the centaur shipper because I've always wanted to make one.
*lil' more backreading*
Inferi. Yes. How many people died in the BoH? Lots o' em. Plus erm you cause go all necromancer on it and make an army of dead animals o.o
*and more, I type slow on my phone okay*
I like mafia, but it seems a bit too rl and may shut people off. Quintapeds though seems incredible. Could you imagine some idiot managing to accidentally bring one back from the island (or maybe even deliberately, for research or something) and it escaping and causing havoc? They have to be mega!deadly if they have the same MoM rating as a basilisk and have their own unplottable island.
*MOAR, MOAR*
Time. God, yes. When Jkr wrote in the time turners she was just asking for a catastrophe. Could you just imagine some idiot trying to do something noble like kill voldemort before the war and breaking time? Though now another part of me wonders about if say such a thing did happen what if the outcome of the war was actually reversed? Though truth be told that sounds like FAR too much work and far too long a plot so you should probably ignore me there.