A couple potential easy options for them.

Do not expand and still hold a championship game like the B12 does. Maybe even encourage some other dead weight to move out of conference so they only have 10 and can play a complete 9 game conf schedule like B12... (Temple, ECU, Tulane, Tulsa - or maybe Navy prefers back in Indy with Army)

Another option go for some quick add options of Indy's in their foot print (don't have to inform a conf they are leaving for football purposes anyway) like UMass, Liberty or Army (not likely but we all used to say that about Navy as well).

Next would be to do what they have always done and raid CUSA. Every single one of them would likely jump quickly at the chance. Or even scrape further down with the Sun Belt teams...

Then perhaps they could try a team from the MAC - but frankly the MAC is above CUSA and not that far behind the AAC as a few MAC teams could easily be close to top tier in the AAC right away and certainly already would be comfortably in the 2nd tier.

Or would AAC try for a big swing and add 3 teams to get to 14 like the 3 big P5 leagues have done. An attempt to further push their agenda as being next up as a P6. That would take strategic teams that boost the profile nationally a lot to even come close to moving a needle in that sphere. Hard sell for sure. I'm not convinced any would really do that unless of course the very unlikely scenario was Notre Dame, BYU and then either BSU or Army. It would take a ND type name with other top Go5/Indy schools to even make the world pay attention to whatever the AAC becomes. Only other thing I could think of is if WVU and TCU or other P5 schools left to join them which isn't happening...

The past attempt at BYU, BSU & SDSU would make more of the country pay attention because more of the country is covered but I do not think it would sway their inclusion into a power conference...