Since everyone's putting forth strange ideas I thought I'd put 
down a few of my own.


Ranma
1 Ranma starts having bizare dreams drawing him to a volcanic island.  
It turns out that there's a dragon's egg hiden there, and it's soul is 
"Incubating" in Ranma.  That is to say, Dragons lay eggs, however the 
souls of the unborn dragons need to develop within other beings, sort 
of acting as passangers and growing as their host grows.  Ranma has had 
his for a while and has grown in ability to the point where the soul is 
ready to return to it's body.  Which means that Ranma has to return it.


Tenchi


Prety Sammy:
1 Rumia's little brother feels sorry for Michelle, and modifies her 
transformation spell so that each time she transformes to and from Pixy 
Misa, she keeps a little more of Misa's vitality.  Naturally both Misa 
and Michelle start getting more than what was planed from each other.
2 The appearance of Tokimi's analog.  Perhapse Earth and Juraihelm were 
origionally the same place, however for unknown reasons they split into 
different worlds.  In the Origional world Tokimi was a queen with her 
two sisteres Washu and Tsunami, however during the split her sisters 
both became part of the newly formed worlds, with their avatars 
constantly reincarnating themselves through different lives.  Tokimi 
however remained in the few places between where the seperation didn't 
happen, and she periodically appears in one or the other world to 
perform mysterious plots which no one else understands.  


Slayers:
1 Partially inspired by some of the debates about the chest sises of 
various anime characters.  Idea:  Lina and Gaury are pursuing a 
fugitive who among other skills is a master of the magical equivalent 
of cosmetic surgery.  He wants to distract them so he can get away, but 
he doesn't want to risk hurting them without knowing it would finish 
the job.  (If their annoyed they'll give up sooner than if they want 
revenge.) so he hits Lina with a breast enhancement spell.  How would 
Lina react to sudenly being better built than Naga?  How would everyone 
react to her differently?

Anime Marvel xovers.

SpiderMan and Bubblegum Crisis:
1 Sylia and Makie attending a symposium to see about new advances in 
Boomer Technology.  There is however a demonstration of Neogenics, and 
an accidentally irradiated spider managed to bite Makie.  He later 
started researching Neogenics and Spiders online, and he discovers work 
being done on synthetic spider silk, and learned about a super strong 
mixture which was flawed by the fact that it evaporated.
2 Nene's mother is actually the daughter of Peter Parker and Mary Jane.
3 A container holding Venom's symbiot is smugled into Tokyo, only to 
break in a ADP raid.

Ranma and Dr. Strange:
1 alt history- after the Jusenkyo incident Ranma and Genma are making 
their way across China when they happen to hear about the Ancient one 
who is talked about removing curses and performing miracles.  They 
decide to try finding him and seeing for themselves.  Ranma winds up 
becoming his student, and a year later when Genma's starting to get 
insistant about them going back to Japan, The ancient one agrees that 
something in Japan is entwined in Ranma's Karma and needs to be 
resolved.