What does Wikipedia know that A.net doesn't? Alitalia744 has been right on past information. While he did not confirm that Delta is intends on purchasing China Southern's MD-90s, he said that it is under consideration.

Delta has retired their 737-200s, 737-300s, and 767-200s. The mainline fleet is being stretched thin. I would like to see additional MD-90s instead of more RJs.

Delta is also rumored to be considering a 100-seat aircraft. Since the 737-600's operating costs are barely any lower than the 737-700, I don't see Delta purchasing any 737-600s. The CRJ-1000 isn't expected to sell to many customers. I would probably say that the Embraer 190 or 195 is the most likely option for Delta. Delta has stated that any new aircraft with more than 76 seats would be operated as mainline aircraft, rather than as Delta Connection aircraft.