I'm surprised that there aren't more DragonBall Z / Star Trek fanfics on the Internet. I went through the few that do exist, and I only liked one of them.
Both series have an alien race of warriors, who are obsessed with fighting and honor (Saiyans and Klingons). DBZ has Frieza, the evil alien warlord who conquers entire galaxies and destroys innocents without mercy. Star Trek has elements of that, with the Borg and the Dominion conquering galaxies. Plus, Frieza and the Borg both use the same naming convention for the races they encounter; there's not much difference between "Frieza Planet 125" and "Species 125".
I kind of want to write my own crossover fic, but I don't have the time. Since we're watching Voyager right now, I'd probably use that. Specifically, I'd do the episode "Scorpion", where the Borg have been attacked/defeated by an unknown enemy. In the episode, it's a CGI monster that never talks. I'd swap that monster out for Frieza. Frieza teams up with the Star Trek crew, in order to defeat the Borg, and drama happens when they realize the only reason he's helping them get to Earth is so HE can get there and blow up the planet.
Frieza can breathe in space, too, which is something I don't think has been featured on Star Trek yet.