Northeastern would work but too many schools are no longer available (Penn State, Maryland, Rutgers).
An original Big East plus 2 additional schools to get to 10 would work but again too many schools won't likely be available (Rutgers, WV, and ACC M7s Miami, VA Tech).
The non ACC M7s should stick together and rebuild the ACC. They would be better off financially for at least a dozen years if not more.
Of the non ACC M7s BC, SU, Pitt, Duke, Wake, GA Tech are all very similar academic schools. They all have small FB fan bases and struggle to compete NIL wise in FB. You also have Notre Dame who fits with those schools in many ways. IMO it is in everyones best interest to commit to each other. All 7 of the above are better off rebuilding the ACC than finding a new home in the B12 or AAC.
The other non ACC M7 is Louisville, who doesn't really fit with the other schools academically and is not Eastern. If they stayed in the ACC you can have a kick ass BBall league. But I think their ego takes them to the B12.
So the ACC will need to add 4 more schools. If you look at a map the FBS playing Atlantic holes are Florida, South Carolina, Virginia, Maryland, New Jersey, Connecticut.
The best available choice in Florida IMO is FAU. South Carolina only has Coastal Carolina and they aren't worth adding IMO. The best available choice in Virginia IMO is James Madison. There are no available full sport schools in Maryland nor New Jersey. In Connecticut you have UConn (for now at least).
If you add those 3 schools and Louisville stays, you are all set with 10 schools in FB and 11 in BBall. If Louisville leaves, then you will need one more school. So looking at available brands and markets your choices are down to USF and Temple. USF doesn't fit with the other schools. If you want to add Temple, you might as well consider Villanova.
Villanova would be the better fit with the other schools, and have the better BBall brand. It also hurts your main TV and talent competitor in Northeastern BBall as the Big East would lose its two most successful programs (UConn).
So IMO it would be best for ND, BC, SU, Pitt, Duke, WF, GT to stick together and add Florida Atlantic, James Madison, Villanova, and UConn.
In FB you actually have a legit shot at making the playoffs. Yes, it is a little weak as a conference but it is still likely to be the 4th best. Is being in the 3rd best with no shot at the playoffs really better?
That is a heck of a BBall league. You have the potential to be the best conference.
It would be great to play everyone in FB and everyone twice in BBall. That is what a conference is supposed to be. You also have good Thanksgiving weekend FB matchups: BC vs UConn, SU vs Pitt, Nova vs JMU, Duke vs Wake, GA Tech vs FAU.
If you go past 10 teams, you can consider going the hybrid route. Adding Army and Navy in FB adds brand value and gets you into MD, a missing state. Then you can have a 7 + 2/2 scheduling model. So over 4 years you play 7 teams 4x (2H/2A) and 4 teams 2x (1H/1A). Plus you get ND close to every other year.
Then you can add non FB playing Georgetown and St Johns. That would completely kill the Big East in the Northeast. Meaning the ACC would control all of Northeastern BBall, which has to add plenty of $ for TV. Scheduling you can go 4 + 4/4 to play your 20 games. So 4 teams H&A yearly, then rotate the other 8 teams with 4 H&A and 4 H or A. Over 4 years you play 4 teams 8x (4H/4A) and 8 teams 6x (3H/3A).
If the ACC M7s do leave in 10 years, how is the above not SU's (and those other non ACC M7s) best case scenario? You are in an Eastern conference with traditional rivals and like minded schools, have a legit shot at the FB playoffs, and have the best BBall league.