The party comes across a magic amulet. This amulet will give each user two wishes, but the wishes almost seem to have a mind of their own.
Missions for any campaign in any city of any realm. As long as crime exists, you will be able to use this submission.
When running a new party, get around the question of plot density by having the first discovery to be a deck of fate, or whatever you call it. If you use a real deck, you should definitely stack it to have the desired outcome, which is to curse the party to attract weird things to happen and involve them. This should not be told to the players at the time, so that they won't just up and try to get themselves 'decursed'. Though quite why they would want to is beyond me.