Uncommon Crossovers.

[stratagemini]

The world needs more Ranma/Sandman crossovers. I KNOW there was one for Ranko's bet. was that even continued?
I mean Sandman is pretty easy to integrate plotwise due to it's flexible nature. And Ranma in dreams is something interesting and new. Of course how to keep the comic nature of Ranma and the flavour of Sandman?
If you're going for something of a different flavour, A Ranma/Lovecraft/Sandman crossover would definitely be interesting.

[Dark Magic]

I agree Sandman is a great comic and could easily cross over with Ranma. But other then the one currently being written by Bjorn and the one your talking about, I don't think there is that many more.

[Delta-Theta]

I would like to see a Ranma/Armitage cross.. heh you can have Ranma take the place of Ross and pair him up with Armitage.. that way you can have Armitage and Ranma-chan in the same clothes ^_^
Makoto-chan/Delta-Theta

[migele]

I kinda like the3 Armitage Idea.
There ain't many good GitS crosses as well.

[Pale Wolf]

Ranma/SM/Heavy Gear.
(Is my Ranko's Bet)
Yes, the few millenia difference on the clock is surprisingly easy to fix

[WG_Writer]

Ranma/Love Hina cross
Ranma as Naru

[Neko-]

Love Hina crosses are out there, so not that uncommon. I never did actually get around to reading one, but can imagine Ranma/Naru exchange may cause some... interesting repercussions.

[Dark Magic]

All the Love Hina/Ranma crosses I've read haven't really been that good. Usually most of them are just Ranma switching places with Keitaro, at least from what I've read.
A crossover that would be cool to see would be Ranma/Real Bout High School. Ranma would fit in perfectly with that school, even though you would probably have to have a Ranma from earlier in the series since Ranma towards the end would be way to powerful.

[stratagemini]

All the Love Hina/Ranma crosses I've read haven't really been that good. Usually most of them are just Ranma switching places with Keitaro, at least from what I've read.
A crossover that would be cool to see would be Ranma/Real Bout High School. Ranma would fit in perfectly with that school, even though you would probably have to have a Ranma from earlier in the series since Ranma towards the end would be way to powerful.

There is one that's good. It's titled either Truth" or "Love" one's teh sequel to the other. it's on FF.net. And Yes Ranma takes teh place of Keitaro, but it's done in such a way thatit's interesting. And If I recall correctly... Godzilla attacks somewhere and is sort of central to the dating subplot. Been A While since I read it.

[Fatuous One]

There is one that's good. It's titled either Truth" or "Love" one's teh sequel to the other. it's on FF.net. And Yes Ranma takes teh place of Keitaro, but it's done in such a way thatit's interesting. And If I recall correctly...

Yup.
It's Truth and its sequel Love by MisatoKitty.
It's unique in that it replaces Keitaro with Ranma, and the rest of the Love Hina residence with the Sailor Senshi. It's also nice that it is a Ranma/Hotaru pairing. ^_^ "Love" is pretty damn confusing though. (Heehee!)

[Raneko]

A Gunsmith Sailor Moon fusion sorta like Gunslinger Moon except more trigger happy.

[Nekomata-sensei]

I was just trying to think up a possible villain for a Sailor Moon fanfic, and through a convoluted thought process I came up with this.
The main villain from M L G Nanoha, Precia Testarossa, _does_ make it to 'Alhazard', the legendary universe the Jewel Seeds and most of the Lost Logia originated from which supposedly collapsed because it's technology and magic became too great.
That world, however, is the Sailor Moon dimension. The Jewel Seeds are artifacts of the Silver Millenium. Precia begins to build a new army of magical resistant robots with the objective of gaining the Silver Crystal, which she believes has the power to ressurect Alicia, while using midchilda advanced magic to cloak herself from Pluto, Mercury, and Mars' detection. Precia subtly directs most of the Sailor Scouts' previous enemies in cannon at them, with the objective of jumping in and stealing the Ginsuzishou given the chance.
After Galaxia, she becomes frustrated, but has now fully recovered her own power after the injuries she took trying to get to 'Alhazard'.
By this time, the Nanoha universe is at where it is at the end of the credits of Nanoha A's.
Precia finally decides to employ her army of magic resistant robots to harrass and hopefully destroy some of the senshi as she begins preparing her final plan, creating a magical device using the Jewel Seeds she has that will be powerful enough for her to defeat Sailor Moon.
Precia's experimentation with the Jewel Seeds causes dimensional reactions that are detected by the Space Time Administration Bureau, who recognize the frequency as the Jewel Seeds that were taken by Precia, and informs Fate, and her friends Nanoha and Hayate since such an investigation would have personal significance to them.
Throw in more plot twists as the story goes on...

[Atlan]

When i first read the title and summery of Gunslinger Moon, i was convinced it was a fusion of Sm and GUnslinger Girl. Though i was wrong, it did give me an idea:
I had thought the plot would be how Ami, Rei and Makoto were found by the agency, and made into Cyborgs. Pluto had owneship of the companies, (being a manipulative bitch) and the Outers, (Saturn being the same age as Micheru and Hakura) were the people in charge of the girls. But when the latest requte, Usagi, shows up, she is imune to the conditioned mental responce, and then starts breaking the other inners out of theirs. Meanwhile, Luna and Artimis have found Minako, training her to take down Setsuna, and keeping her free of the clutches of the Agency and the outers.

[GenocideHeart]

I'd sort of like to see A Ranma/FF Tactics crossover.

And I just wrote one, although with Sailor Moon involoved as well. Coincidence? Look for the first chapter in the C&C section - it's called The Zodiac War.
In general, FFT has precious few fanfics. And most stink to boot. Pity...
Regarding crossovers, I remember seeing a Ranma/Rozen Maiden xover titled Lehmmaiden on FF.net. It was good, give it a spin.
Other crossover ideas, fairly random:
-Sailor Moon/Tokyo MewMew: just because.
-Ranma/Shadow Hearts: what if Ranma had the same power as Yuri Volte Hyuuga? Even better, what if Ranma was Yuri's reincarnation, and one of the fiancees (or possibly another Tendo sister) was Alice's?
-Ranma/Suikoden II: Yes, it has been done before... but I actually was thinking about NOT giving Ranma a True Rune. Instead, let him meet the Suikoverse's martial artists - Pahn, Long Chan Chan, Wakaba, Oulan... Hell, it could set up a Ranma/Wakaba pairing. Wakaba likes strong people, and is overeager to learn. She'd start following Ranma simply to learn his tricks, and Ranma might even be willing to exchange a couple of his own tricks for a White Tiger rune and lessons on how to use it.
-Ranma/Makai Kingdom: we all know Ranma's cursed form has red hair. Well, what if the Drowned Girl was actually Overlord Zetta's long-dead daughter? Imagine the scene: a bunch of Overlords suddenly show up in Nerima looking for Ranma, followed by Zetta himself - and the resulting fight between Zetta and the Overlords that are looking for Ranma may end up leveling most of the ward.

[migele]

In my ideas Folder:
Ranma/Nanoha As: I fainally managed to fix a plotline. poor Ranma.
Sailor Moon/Nanoha: Boost SM in time and let Nanoha, Fate and Hayate make a trip to Juuban where the monster of the week tries to attack them. Velka Type meet Silver Type.
Ranma/MAhou Sensei: haven't seen too many of those.
Ranma/Miyuki-chan: Blame Yarrow for that one, is being written when I have time.
Things I want to cross but I can't get to work yet without trouble.
Rozen Maiden/ Sailor Moon
Juuni Kokuki/Ranma
FLCL/SM: I'm still wondering if this needs a plot and any sense.
.hack/SM: Replace Tsukasa?
well those are the ones that came into my mind at the moment.
I think the fic with the most promise at the moment is a Ranma/Tsukuyomi Moon Phase cross that I'm working on, slowly!