The easy answer is - whoever ND wants.
In reality, it depends on the what the conference thinks is most important. It is looking to secure the Northeast/East Coast? Is it looking for new markets? Is it looking for a basketball or football power? Can it raid another P5 conference (XII) or does it need to look to G5 conference? Does the conference care about academics?
Texas sounds great, but has shown that it does not play well with others. The XII lost three teams (allegedly) because of Texas and its hubris and the invitation to join the Pac fell apart because of the refusal to incorporate the Longhorn Network into the Pac Networks. If Texas cannot learn to be an equal member in the ACC, I am not sure they are worth it.
If Texas is out and the conference cares about academics, WVU and Cincy would also be out. What does that leave? I think the ACC may have to look at the AAC. From a university, as opposed to an athletic perspective, I would like a private school with strong academics. This would rule out UConn.
Although not an athletically in the same league, I would like Tulane be considered if there is a contractual requirement it invest further in , athletics. It would be a great road trip, fits in academically, and gives SU another private school ally when voting on league matters.