Or, a Star trek adventure that turns into Aliens (although the whole SNW Gorn arc did this already, so it wouldn't be as much of a fun surprise).
Trek has so many crossover eps (TNG, DS9, VOY) that not much is left.
And SNW has covered a bunch more. It's easier to say that Trek is ALL crossovers these days... its core identity is so weakened. Especially in SFA... three eps in, and 2 are outside the space operatic core genre.
What fun mash ups have you done or would you like to do?
Trek: Military ground actions, horror, political actions, courtroom drama, law enforcement, time travel, special operations military. (Prime Directive 1e is "SEALS in Spaaaace!"). Even a Musketeers crossover once.
Traveller (counting Military ground, military space, trouble shooters, and merchantile as core): Horror, time travel, Economics/board room drama, courtroom drama, Medical mysteries... and that all from published stuff. The Tron crossover was a flop. Next time, I set the disks to 6d6 damage.
D&D: Like trek, it's hard to define what's crossover genre-wise. I've done most of the same genre mix-ins as Traveller and Trek. The older I get, the less I enjoy D&D, tho', so calling them Fun may be a stretch.