Having run Champions and Ars Magica games back in the day, one my favourite minicampaign types was was "Unholy Alliance" - the classic one where our heroes are forced to work with villains (or really, really irritating people - as a purely random example, Harry working with Morgan). They may be working under duress, against a common/greater foe, or even just with no-one else to turn to. But with all the opportunities for snarkage and octuple-crossing plots, it's good fun and a lovely way to mess with people's heads.

Another favourite requires a player's (knowing or unknowing) co-operation... "Seduced by Power". A player acquires a talent or object that is WAY too powerful for their characatier - and while there's benefits for the team and the player, (s)he may be forced to choose between letting go of their newfound strength - or letting go of their friends/health/sanity. This has worked well in AM, Champions, and even MERP...