Maybe a small sucking sound (i.e. loss of transatlantic routes)...but, other than that, there are a few reasons that CLT won't go the way of PIT, IMHO.
1) Currently, US & DL move the vast majority of NE/SE traffic through these two hubs. That traffic won't go away and, logistically, ATL can't handle the growth.
2) Unlike PIT, CLT is about the cheapest hub to operate anywhere in the nation. It is a cost advantage for US - not a disadvantage.
3) US dominates CLT. The "new" Delta would dominate CLT and ATL - the only major connecting hubs in the southeast. This is a major advantage (and probably the biggest anti-trust argument against the merger). There is no reason to believe that Parker & Co. wouldn't recognize this and would simply give away this huge benefit.
If this merger goes through, I believe most of the CLT operation is safe.