.... win your conference title, you're in. Simple as that.
There would always be controversy as to who is left out of the at-large teams, but the #9 and #10 teams complaining about having a legitimate shot at the NC in that scenario wouldn't have nearly the same argument as the #5 and #6 teams complaining today.
Only problem I see with 8 teams is possibly playing 3 playoff games, which might have some people clamoring to drop the regular season back to 11 games.