The reality is that when you add three more teams to a conference , you have to do some shifting around. Some of those long time rivalry games will have to go by the wayside. It's unavoidable. I'm sure FSU, Clemson, UNC, UVA et al understand this.
The problem you have to consider is that at least currently there are very few high profile matchups to be made in ACC football. It's one thing to lose the Tennessee-Alabama game or Georgia-Auburn in the SEC, because while those mean something to fans of those schools, nationally, their replacement game is likely to be of at least as much interest if not more.
The ACC doesn't have that luxury and won't yet for awhile. They cannot afford to touch the FSU-Miami or FSU-Clemson games. The only way is if they were replaced with a VT game, but nobody is even thinking that way. It is a problem already that now with 14 teams you won't get Clemson-VT or FSU-VT or Clemson-Miami but once in a blue moon.
Now, UVA-UNC? I think that can be on the table in the scenario you describe, because like the SEC example, that's replaceable as far as the national interest (TV interest) is concerned. I am not saying FSU-Clemson is a superior as a tradition or anything like that, it's just dollars and sense right now. Again, given time it could change.
I know there are a lot of problems with it, but I keep going back to a North-South split and dropping the permanent cross-division game. And drop the home-and-home requirement for the cross-division. Playing two of seven accross the division would at least cycle you through in 3.5 years. Yes, it would still be forever until you got them in your house, but you would have that Miami-Clemson, FSU-VT game every 3-4 years.
I don't think it's a secret that a lot of the teams that would be in the North division could use some help to get on (or back on) the national scene, I don't see a big downside if they have a weaker division that helps them emerge. With a better than average team, a good cross-division draw, and a couple bounces, a team like Syracuse or Virginia could go 11-1 or 10-2. A season or two like that could be a huge game changer for programs like SU, Pitt or UVA.
And it's not like playing Louisville and VT and two from a pretty stacked South division is a total cakewalk. Just because a North-South is lopsided today doesn't mean it always has to be. The SEC East was mostly a joke when those divisions were set up, and they dominated for about a decade.
I think if you make the setup based on geography and historical rivals and maximizing matchups for TV, it ultimately will pay off more than trying to artificially shoehorn divisions together based on perceived program strength as of today.
North:
BC
Syracuse
Pitt
Louisville
VT
UVA
Wake
South:
UNC
NCSU
Duke
Clemson
GT
FSU
Miami
And I say that knowing that it makes FSU's road to an undefeated season more difficult.