Delta has been looking at a 100 seat aircraft for a couple of years now. They know Boeing is going to follow on the 787 with a 737 sized replacement using the technology from the 787 program. Like the 787, the new Boeing small jet is percieved as a game changer.

Delta has also been seriously considering the E190 from what I've read, but they don't like the performance of the aircraft. They don't like the ergonomics of the CRJ as a mainline jet.

So the best option seems to be to wait for Boeing. However, Boeing is waiting on GE and the other engine manufacturers and the 737 is still selling well. Delta has 10 737-600's coming, which is a 100 seat jet.