Recently, I paid tribute to the great team-up comics of the past - and Batman: The Brave and the Bold animated series by ricochet - and it made me think about realizing team-ups in role-playing games. Because you know what a major problem with lots of licensed RPGs is? Splat books. We buy lots of sourcebooks full of character stats, but players would rather create their own characters than play them. The team-up campaign or one-off is perfect to maximize your RPG collection! Pretty good for small groups of players (one GM and 2 players is actually optimal). And while the following article is would seem dedicated to superhero RPGs, its ideas can easily be adapted to other licensed games like Doctor Who and Star Wars, or with even non-licensed games like D&D or Top Secret.
The full-on team-up campaign
In this campaign idea, one player always plays the same character (Batman, for example) while another plays the guest-star of the week. To achieve the true madness of the team-up comic, the guest should be picked at random, either from a table or out of a stack of character sheets. Rolling on a table could help successive games achieve a certain balance, as perhaps a first roll could have a slim chance of sending you to a villains' table (for when the Doctor has to partner with [roll] the Rani), or limit the number of times Batman has to team-up with one of DC's cowboy stars (present day characters have higher % on the table). Alternately, there is no recurring star, and all players choose new heroes each session. Or you could play them in a chain - one stays, the other changes, alternating like that each session and allowing a player two sessions per hero. The GM could allow his players more than one roll/pick and the chance to choose the favorite of three characters, and could roll on a villains' table as well, for extra randomness.
Obviously, these picks should be done well before the session (say, at the end of the previous game) to give the GM the chance to meet the challenge of crafting a scenario that could realistically involve these varied characters, and the players a shot at researching their choice on Wikipedia or in the original source material. Can't expect everyone to be immediately conversant with Kamandi, Robin Lefler or Elminster, after all.
The team-up one-off
If that sounds extreme, you need not go that far. The self-contained team-up may be a perfect distraction in between larger arcs, or when only a couple players can actually make it to the game. Maybe the team-up is with one of the original Player Characters, maybe it's just a way to deepen the game setting by playing what would normally be NPCs.
Obviously, the DC, Marvel, Who, Trek and Wars universes are big enough to provide an almost limitless number of permutations, but that never stopped Batman from crossing over with Daredevil, Predator and Judge Dredd! For extra fun, allow for these to happen on occasion. One trope you should respect when doing so is to include a threat from both properties. For example, Doctor Who and Star Trek TNG are crossing over right now, and feature the combined menace of the Borg and the Cybermen.
TV Land: Busting out of canon
The ultimate cross-company crossover, of course, is when you throw everything in the same genre into the same universe regardless of whether or not they could normally meet on the screen or page. If you're playing a spy game like Top Secret, for example, your game could team up Bond & Cinnamon Carter, or Chuck & Warehouse 13, or Jason Bourne & the Prisoner. Doctor Who is a great vehicle for such things, since a TARDIS (or other temporal means of travel) can have a character meet any TV or movie character from across time and space. Romana and Sherlock visit Eureka under threat from Terminator robots. Adric survives Earthshock and meets up with Captain Caveman or the guys from Quest for Fire before popping out of a Primeval anomaly where he helps Oz from Buffy escape a version of Oz the prison run by the Wizard of Oz.
Go crazy, it's what the team-up experience is really about. Also, non-sexual fanfic.