From: 	RPM - acct 3/5[SMTP:rpm39788@Bayou.UH.EDU]

        Is there any guidline on new characters or what?

Let's see... from what I know...

SAOTOME, RYO
series: Descents and Inversions
info: son of Ranma Saotome and Ukyo Kounji.  Looks extremely like >Ranma.

>From the same series: 
KUNO TACHI
info: son of Kuno and Nabiki. Well dressed, well manered but materialistic young man.

KUNO TACHI
info: daugther of Kuno and Nabiki. Smart and greedy like her mother.

SPRING
info: daugther of Ranma and Shampoo. Has same pigtail as her father. Has waist-length purple hair.

SAOTOME, RANKO
series: Careful Destiny
info: in an alternate universe, Ranma Saotome's sister.  Looks like

      Ranma-chan, except with black hair.  Trained in Anything Goes.

And 
SAOTOME RANKO
series: Martial Arts lemon
info: Ranma's twin sister, looks like Ranma-chan down to the red hair. Has nannicuan curse.

and
SAOTOME RANKO
series: Descents and Inversions
info: daugther of Ranma and Akane. Same looks and curse as above.

Not to mention one more in "Future Ranma". Another daugther, mentioned in passing.

Others:  there was a fic with three Saotome Daughters in it....

They are:
SAOTOME AYUKI
info: daugther of Ranma and Shampoo. Looks like Ranma-chan with purple hair.

SAOTOME KEIKO
info: daugther of Ranma and Nabiki. Looks like Nabiki with longer hair.

SAOTOME MIDORI
info: daugther of Ranma and Ukkyo. Mixed features of Ranma and Ukkyo.

As you can see, a good number of them are not just half-siblings, but full-siblings from different universes. Think of the chaos when they get togther!