I'm personally not a big fan of Inuyasha, so I only have a couple for you, none of which are ideal for the situation you are looking for.
First up is:
Shadow's Rebirth by Xen-Oh which has the current arc (though it's an effectively dead story) taking place in the Inuyasha world in the past. To get there though you have to sort through a multicross with the primary parts being Sonic and Sailor Moon. It's a pretty decent story, but there are a ton of references that you might not get if you aren't conversant on a bunch of different series. Ranma winds up as a cat hanyo, I think, though the exact percentage of his demonic side is unsure.
And the other I've got a link to off the top of my head is
A Third Chance at Life by Lumias which features Ranma as bearing Kikyo's soul. It does not go into the past though.