I know any expansion without ND is a non-starter, but let's play make believe for a minute or two...
let's say that ESPN decides to open up the checkbook for the ACC, network and all. Let's also say that as part of that deal, they want the ACC to expand their footprint for more subscribers (also assume ND is still independent and all GORs hold, meaning that the 2 additions come from the G5). To do this, they add:
1. Houston: Large public university, lots of cable subscribers for ACCN. Let's be realistic - Texas and aTm rule that state. To bolster Houston's profile, ESPN brokers an annual rivalry with aTm (i.e. ride aTm coattails to elevate Houston's profile/status). Since Texas and aTm no longer play, this offers an in-state rivalry that could/should attract a great deal of fans given the size of both schools. It's a bit of a geographic outlier, but 2 enormous airports in Houston should ease travel concerns.
2. Memphis: Large public university, >60K stadium. Not as many cable subscribers, but a great new recruiting area for the ACC. Similar to #1, they are overshadowed in their state by Tennessee. ESPN could broker another ACC-SEC rivalry with either Tennessee or Ole Miss to ride SEC coattails to elevate their profile. I assume Memphis has a decent airport to ease travel concerns.
Granted, ESPN already owns the states of Texas and Tennessee, but this would offer more subscribers for the ACCN. What I like about these two is that they both have potential to compete in a P5 conference due to their location, recruiting ground, large enrollment and alumni (i.e. not private schools). It also uses (or at least attempts to use) the popularity of SEC football to help elevate ACC football. Also consider now nicely the North - South divisions shape up:
North: BC, Syracuse, Pitt, Louisville, UVa, VT, UNC, Duke
South: NCSU, WF, Clemson, GT, FSU, Miami, Houston, Memphis
9 conference games, no permanent crossover rivalries.
The only sacrifice is the NCSU-UNC game, which could be played OOC since neither has a permanent OOC rivalry. Market it as the rivalry beyond the standings or some other catch phrase. It preserves UNC-UVa (the oldest rivalry in the south) and allows us to play everyone in 4 years.
If you read this far, then I apologize for taking those few minutes of your life that you'll never get back