It's not "Utopia," for sure. "Utopia" was about demons ruling the future and humans being little more than slaves.
I think I know which story you're talking about, I know I've read it, but I can't remember the name.
When she got involved with Sesshoumaru at the beginning of the story, he was very different. He had lived through being overthrown and humiliated, and (if I remember right) either blamed some of it on Kagome, or was more determined to control her as a result of the torture he'd gone through.
She and Shippo escaped to the past when Sesshoumaru went a bit nuts, and found InuYasha and Kikyo, and their kids, and started living with them. Then Sesshoumaru showed up and they figure out that he's gone through all that torture and stuff, but Kagome and Sesshoumaru somehow work it out and get together. They start to re-build the West, with a new castle or shiro, or whatever, and fight off attacking foreigners (or something like that). Kikyo dies, Inu goes a bit nuts and takes off, and so on.
Sound like the right story?
~~Wiccan~~