In-universe crossovers

The thread on the Star Wars cross over reminded me of a pet thought experiment of mine (which could lead to interesting, if funny, adventures), which is to imagine cross overs with other universes while strictly sticking to Star Trek canon. The Trekverse is rich enough to offer many possible ways to faithfully reproduce other universes, without even having to create new races to meet the requisites.

A few examples :
- Stargate: Have 21st-technology world discover an Iconian portal on its planet. This world may have been visited some time ago by the parasites from Conspiracy, who particularly enjoyed this planet's inhabitants physiology. You could throw in some god aliens from the Apollo people too. In this cross-over, the UFP would be the rough equivalent of the Asgards.
(This is not the first time I mention this crossover - the Apollo being a suggestion from Tatterdemalion King IIRC - and I discovered that someone posted it very similar on the Adventure Seeds thread a long time ago).

- Highlander: Flint the immortal. Maybe he was the one to win the Prize, who knows.

- Doctor Who: Either the Traveller's people or the El-Aurians are good candidates. Both don't seem to mingle much with other races, save for a few individuals who like to discretly mix with them (for good or bad). Both have a sort of time sense to an extent, and for the Traveller's people some way to travel through a lot of dimensions. El-Aurians are very long-lived and have been wiped by a ruthless ennemy. Building something Whovian from this does not seem too far-fetched.

- X-Men: There is a early TNG episode where the crew encounters a being from a planet where some individuals are reaching the next step in evolution, and developing superpowers as a result (the guy turns into energy or something like that in the end). They are even persecuted by the other inhabitants of their planet.

Other suggestions ?

All Our Yesterdays featured a few alternate Trek realities based on 'what-if' situations... such as the Borg assimilating Sector 001 and the betrayal of Kirk & co at Camp Khitomer by his former first officer - an Andorian - who attacks them as part of Cartwright & Chang's plot.

It's my favourite part of that sourcebook, because it shows how certain key events could have played out into a whole other reality. :D

(scratches his head)
well how about
Andromeda: you could have either a SF ship stuck in the event horizon of a black hole emerging after the FED fell in the 32nd century. Or, have a ship of a previously unkown/little known or understood galactic power from an earlier era find it's way into a Trek time period. Either way, the noble captain of the ship forges alliances to try to bring back a better period for the galaxy.

Avatar: a pre-FED earth finds a good source of dilithium, but the planet's inhabitants don't want us there. Sympathetic humans help them, and hope other civilizations (notably the Vulcans) will pressure the industrial powers that seek to strip mine a world to back off and let the humanoids that exist there to live in peace.

I haven't seen much of this season, but I had an idea that a Trek-verse version of Fringe might be an early front in the Temporal Cold war, with 21st century humans making surprising breakthroughs in technology, possibly with temporal help. Also in the mix are humans trained by some alien society to keep our development on track (Gary Seven, Eight and Nine, perhaps?). And who is the mysterious genius from Carbon Creek, PA? And why is he at the heart of all of it?

This one is a little sketchy: borrowing from some summer block busters and one big untouched bit of Trek movie information.
A group of explorers from the Machine Planet head to the Alpha Quadrent, curious as to what happened to V'GER. Realizing that this part of the galaxy(universe) is dominated by organic life, they adopt forms that blend in with our society: smaller robots create false humanoid forms to 'pass', while larger entities are rebuilt to look like ships, space stations, etc. The come to study us, but some fear that organics are a plague that will eclipse technoforms like themselves (and maybe the don't like how AQ races have 'enslaved' technoforms to do their bidding!), and a secret war breaks out amoungst them!