I've read a ton of fanfics and am normally wary of Ryoga being a centerpoint character.
(Especially after a trainwreck Ranma/Star Wars crossover that was so terrible that it was impossible to look away from...it did predict the return of Darth Maul however. Imagine Darth Maul training Kuno to beat Ranma while Dooku trained Ryoga to do the same.) (FF.net pulled the fic because it had a bit too much violence and lemons.)
But your introductory chapter here has the signs of being a great fic. I can see Ryoga taking the effort to care for Baby Harry while at the same time learning more about the world. Also, the possibility of there being a cure for the Curses in magic would make Pig-Boy a juggernaut.
Just one thing that is kinda scary: Happosai with access to the Invisibility Cloak or Disillusion Spell.
My vote for the pairings:
Harry/Hermione (Ron/Hermione seems to be a disaster in the making. And I really don't like Ginny.)
Ryoga/Ukyo (The two of them seem to get along much better than any other couple in the Ranma 1/2 series. Also, the Pig Farmer and Cross-dressing Ninja made no sense. In Ranma 1/2 that is saying something)
Ryoga/Shampoo (Strength & Guile in one couple.)
Harry/Luna (For an extremely large serving of chaos, add the insanity of Nermia to the craziness of Luna Lovegood, Straight-Jackets are suggested for everyone's safety.) I can actually feel a bit of pity for Happosai & Cologne if this occurs, that chaos would be titanic.