I don't normally do fanfiction, but the well-known reviewer SFDebris penned an epic Star Trek/Star Wars tale a number of years ago, before his video reviews took off. It's worth a read, or at least a boggled, disbelieving stare. http://sfdebris.com/inprint/unity.asp
there's a few out there, I even saw one that crossed Voyager with Back to the Future, Kingdom Hearts, Buffy the Vampire Slayer, Ghostbusters and Doctor Who, at the same time. Although there are some good ones, these generally fall short on quality because they are so mish-mashed.