They don't really have many options for used aircraft.
I have a hard time believing they'll introduce a new type, or that they want a 73G-sized aircraft. Which leaves 738 ,739, 319 and 320s.
I don't think there are many 738/9's available. 319s and 320s *may* be, but for what they'd want, you'd be looking at quite old build aircraft.
DL has been very creative in what they've been up to in the used AC market. They haven't been going after the primary workhorses, the 737 and 320, but rather, the bin-ends, and putting them together.
Most airlines didn't even have the 717 on their radar. DL, with extensive experience with the MD80/90, was suited to add the '95 to their pack of mad dogs. WN, wanting consistency, didn't really want to keep their 'oddball' fleet, so they leased it to DL for cheap. What is UA going to do? knock up HA for their 18 717s? I don't think so.