It would be hard to get a 12th football member for 2020 unless you dip down into the less desirable schools like Marshall, FIU, FAU, Southern Miss., Georgia St.
I just looked at BYU's future schedules. It seems like they really couldn't join the AAC until 2024 at the earliest. In 2020, they have 6 P5 schools scheduled (Utah, Mich. St., Arizona St., Minnesota, Missouri, and Stanford) as well as Houston, Utah St., Boise St., Northern Illinois, and San Diego St. If you are undefeated or a one loss team with that schedule, you are probably going to the playoffs.
Army is problematic because of the Army/Navy game played in December could not be a conference game. Due to the history of the Army/Navy game, and the recent success of Army playing a lesser schedule than Navy, I think it would be impossible for Army to become a conference mate with Navy.