Then you need to define "big". Pitt won their division and played in the ACC Championship Game, which would qualify as a big win unless you insist on using the beauty contest rules that a playoff system generally strives to eliminate. The problem with the beauty contest criteria is there is not a legitimate path for a G5 team to make the playoffs because program quality tends to change over time. For example, UCF had a game against North Carolina cancelled this year due to a hurricane. 2-9 North Carolina would not likely qualify as a "big win" using your criteria and there were several stories written where not playing this game would strengthen UCF's schedule. The game was scheduled in 2016 when UNC was ranked #22. (BTW: if nobody other than Clemson is a "big game" in the ACC, then Clemson also failed to play a "big game" this season prior to the post season. Their biggest win was by 2 points at Texas A&M who lost 4 of their first 7 P5 games while being outscored 183-156. Texas A&M would be unlikely to qualify as a big game for UCF).
UCF plays Stanford next year and the game was scheduled in week 2 of 2014 when Stanford was ranked #11, but Stanford finished this past season ranked #32 in the Coaches Poll. Is Stanford a big game next year or is it too early to tell?
Last year UCF had a game canceled against Georgia Tech due to a hurricane and the game is rescheduled for 2020. The game was scheduled before the 2015 season when Georgia Tech was ranked #16 and later climbed to #14 that season after defeating Alcorn State and Tulane. Is Georgia Tech a big game or is it too early to tell?
If Stanford, Georgia Tech, UNC, and / or Pitt are not "big P5 games", you've reduced the "big game" field to about 10 teams or less (Alabama, Clemson, Ohio State, Oklahoma, Georgia, and perhaps Michigan, Texas, Penn State, LSU, Stanford, USC, Florida, and Florida State). Florida refuses to play UCF in a home and home because they insist the distance between the two programs exceed the 125 miles that separate the two schools and Florida may not even qualify as a "big game" based on your criteria (ranked in the top 10 for two weeks in the past three seasons). They wouldn't have last year after finishing 4-7, correct? You're essentially advocating the creation a system where a G5 team needs to take a paycheck game where they might get paid more like an FCS opponent than a potential contender in hopes the team doesn't tank between the time the game is scheduled and played in order to have a chance at the playoffs. Once you appreciate the fact that you've eliminated all chances that a G5 team can make a four team playoff, the only reasonable option is to expand to eight. Unlike your Princeton reference, UCF is in the same football division as Alabama.
