I got a couple more ideas, these ones though are Terminator: The Sarah Connor Chronicles crossovers with Firefly/Serenity. I have a bunch more other TSCC ideas, but I just typed them up somewhere else and don't want to copy them over here right now, maybe later though.
Crossover with Firefly/Serenity
-John(15) accidentally is sent to the future.....like way way into the future.
-He is eventually adopted by the Tams. There is no River, or you can think of John as River as he'll basically be her....if that makes sense.
-John is accepted into the school that River in the show was, and like in the show John and Simon are extremly close.
-Simon breaks John out of the school, they find their way to Serenity, yada, yada. Basically everything that happened on Firefly happens the same but with it's a teenage boy named John instead of a teenaged girl named River.
-So, John is 15, adopted by the Tams, spends around 6 months or so living with them till he goes to school, spends 6 months to a year there before Simon breaks him out. Then they spend about 6 months on Serenity.
-After around those 6 months so John like River is now around 17ish, Sarah, Derek, and Cameron manage to travel to where John is. You decide how they find him, if they landed on a planet and luckily ran into the crew or they landed on the ship or something.
-Since this is also a crossover with not just Firefly, but also Serenity, they get there basically right after the thing in the bar/tavern happened.
-After Simon explains whats going on, Mal demands they explain whats going on, who they are, and what they're doing on his ship.
-In return they explain/demand further explination and to see John.
-Everything in the movie happens, except no one(Wash) dies.
-John, Derek, Sarah, and Cameron find a way to get back to the past, if you want the crew can come along.
-Once back in the past they have to try and help John become well, not insane. Maybe a good time to bring Charlie in. =D
-Things continue, but Idk what they are this is where you take over so to speak.
-Also couple more things, just not sure where to stick them so I'll put 'em here. While John for the most part has the same story line, meaning he does basically everything she did, he also adds to it a bit and what not. Like maybe he in his insanity talks to the crew about his future and stuff. Everything thats happened to him, all that good stuff. John never told the Tams the truth about where he came from.
-If you want you can have Sarah, Derek, and Cameron show up before Serenity takes place.