If the Big 12 were to switch to an 8-game conference schedule, I would prefer to fill the extra game with an upper tier P4 team. The only way to get into the conversation as to whether we're a top team in the country is by playing the top teams in the country. Padding the schedule with mid-level G5 teams or FCS teams may work for the SEC because of the level of talent in that league, but it wouldn't work for Big 12 which still needs to establish legitimacy.
To be the best you need to beat the best.