Richard Robinson wrote:

Who said his father was still Genma? change part and you might change
all. A lot of the old gods gave their children to mortals to raise. So
life from Ranma's and Genma's and even Nodolka's points of view
wouldn't have to change but the future from then on becomes wide open.
 As with everybody else I would like to see sequels to this tale of
magic and motherhood!

I'm inclined to think that the Universal Force would avoid altering
history more than it possibly has to in order to fulfill a contract.  
Which is why Ranma's life probably stayed pretty much the same until
he "discovers" that Urd is his mother, and comes into his "inheritance".
Besides there are no other really handy candidates for fatherhood,
and Urd and Genma aren't that incompatible, at least over the short 
term.  They are both gifted, self-indulgent impulsive people.