Okay, I've got my ideas for crossovers:
Star Trek/Firefly: the
Enterprise crew comes across a star system with dozens of planets and hundreds of moons, settled by humans who speak most of the languages of Earth-essentially, a collection of lost (unknown) Earth colonies that had resulted from the mess of the 3rd World War and its aftermath. They are feted by the government of the Alliance as having come from what the Alliance calls 'Earth-That-Was' and are shown how advanced the society is despite not exploring out of it due to not having warp drive, but a chance encounter with the crew of the
Serenity make them realize that the Alliance isn't what it's cracked up to be, leaving them disappointed. Before the
Enterprise leaves, Kirk makes a speech about the Alliance and how it's advanced technologically, but retrogressive sociologically, and that before it can be a part of the Federation (and
if it wants to be a part of the Federation), it has to change.
Star Trek/Venus Prime-A tear in the fabric of space & time sees the
Enterprise (from the 2009 reboot) end up in an alternate universe where the people of Earth have settled all or most of the solar system, but have not left it yet because the technology is centuries less advanced than that of the Federation in the prime universe (the time/space thingy deposited the
Enterprise not just in an alternate universe, but in the alternate universe's 22nd century where faster than light travel has not been achieved yet, and with spaceflight tech similar to the tech of t
his movie.)
The Enterprise's arrival causes a lot of commotion on Earth and the rest of the inhabited system (run by a governmental entity called the Council of Worlds that's similar to the United Earth Republic/United Nations of Sol), with a lot of social upheaval, especially as far as a cult called the Free Spirit is concerned.
The Council of Worlds enforcement arm, the Board of Space Control, assigns Ellen Troy (assumed name of Linda Nagy) to deal with them, her friend/associate Blake Redfield assisting her. A transfer of 23rd century tech to the peoples of the CW happens while the
Enterprise is in the midst of getting repaired at an orbiting BSC spacedock; Kirk & Spock determine that allowing this wouldn't be violating the Prime Directive or the Temporal Prime Directive. The
Enterprise crew, Lt. Troy, Redfield, The Commander (Troy's superior) and Professor J.Q.R. Forster must not only stop the Free Spirit cult from continuing what they were doing as seen in the original
Venus Prime novels, but also deal with the discovery (on the Jovian moon Amalthea) of an ancient alien world-ship, which may help the
Enterprise and its crew get back to its own time and dimension.
The
Enterprise crew assists the expedition on board the
Michael Ventris to research the world-ship, and subsequently the
Enterprise itself is pulled into a stable black hole that the world-ship creates, its ultimate goal to be involved in an ancient interstellar conflict. After the conflict is resolved (with a lot of exploring of the solar system
millions of years in the past!) due to the involvement of the
Enterprise and its crew, as well as the apprehending of William Laird (the villain of the
Venus Prime books responsible for how Troy/Nagy is in them), both parties go their separate ways, with the
Enterprise crew being able to convince the alien to help the
Enterprise get back to its own dimension, space, and time, and the crew of the
Michael Ventris doing likewise back to the 22nd century of this dimension,
minus Troy/Nagy and Redfield, who were so altered by the experience (partially caused by transforming themselves to be able to breathe water in order to communicate with the alien) that they elect to stay with the alien.
Star Trek/TRON Legacy-Starfleet discovers that most of the computer technology used by it is based on tech invented by ENCOM, an old corporation from the 20th and 21st centuries, and that some of this tech may have generated artificially intelligent 'beings' called ISO's. Discovering that Kevin Flynn (the discoverer of the ISO's) had actually been digitized into a computer system and then discovered the ISO due to his experimentation and tinkering with said system ( the 'digital frontier' of the two
Tron movies) Kirk, Spock, McCoy and Uhura manage to be digitized and sent into the computer system, having an adventure similar to that seen in the two
Tron movies.
