Oregon will fill that void.....starting this year.
This may the the year of the Duck!!
Perhaps, and maybe even probably, on-the-field, but it will take more than one or two years for someone other than USC to garner national attention and interest and respect before anyone outside Washington, Oregon, California, and Arizona pay too much attention to Pac 10 football.
Do you really think the average college football fan in Virginia or Kentucky or Florida will look at their TV schedule and see Oregon v. (name your Pac 10 opponent that is not USC) and say to themselves "I have to see that game"? Especially if said fan knows USC is ineligible for the title (despite probably being the best team in the league) or a diminished program due to probation signaling the league is even less relevant than when USC was the only national capable team.
Heck, Oregon lost the Rose Bowl to THE Ohio State University in convincing fashion last year. And the Big Teneleven champion has been the beeotch of Florida and LSU and Texas in BCS games the prior three years.
Over time Oregon or UCLA (who probably has the second best national reputation in that league despite never winning a national title) or someone else may step forward and make their mark nationally. But, in my opinion, it will take a decade or more of sustained championship level play and with USC not being viewed as a lame duck program. Perception is a very powerful obstacle, as I tell many clients trying to reposition their brand.