In order to really start a rivalry, you have to play, win, and lose something important to both you and the other team. This is why CSU isn't a rival - it's (really) annoying to lose, but we can lose that game and still have a successful season. The converse doesn't appear to be true; simply put, for some reason the game means something to them that it will never mean to us.
Coach Mac declaring NU a rival didn't make them a rival. What made them a rival was the fact that when we played then we won and lost something important to us, and they won and lost something important to them. And it happened almost every year for 22 years.
For the last 8 years of the Big 8, the conference championship depended on the outcome of that game (both teams couldn't have won it every time they played, but at least one team could have lost it that day), and similarly, for 12 of the 15 years the teams played in the Big 12, a berth in the championship game was on the line for one, or both, teams. And for at least 6 of the games over that 23 year span, a possible national championship was at stake.
They became a rival to us because they denied us something we desperately wanted, and an honest fusker can will admit that we became a rival to them when we denied them something they desperately wanted (it killed them that we won an mnc during their multi decade drought).
We won't develop a true meaningful rival in the PAC until we get to be very good again. And it will probably require another PAC team being very good at the same time.
I pretty firmly believe it will be SC. I don't think the ducks will be good enough, long enough to for a true rivalry to develop. It could be UW, but I don't think both teams will make the championship game frequently enough for it to become a grudge match.
But once we beat out SC for a berth in the championship game a few times, and they keep us out a few times, I think the hate will begin to flow. And on both sides. They'll always say UCLA is their "main" rival, but we're big enough assholes that they'll start to hate us as soon as we start seriously winning again.
TL,DR: CSU is not and will never be our rival. NU was our rival because both teams were so good for so long. SC will become a rival for the same reason.