I too wonder about narrowbodies. I expect some mid-size used orders, but I also expect something to replace the MD-80/90, 717, and oldest A320/737 in the fleet.
DL will take delivery of 22 A321 this year. These were the de facto replacements for the MD-88 and MD-90 (in other words, the aircraft scheduled to enter the fleet by the time the MD aircraft exited). DL has 125 A321NEO scheduled for delivery within the next several years. These could be used to backfill capacity as needed, replace the older A320 and B757, and for expansion. And of course, I'm certain there's some merit to the rumor that DL's looking at second-hand late-model 739, which could be used for a similar purpose. The oldest B738 has awhile to go before it's retired (5 were built in 1998, 11 in 1999, 22 in 2000, 27 in 2001, 8 in 2002, 2 in 2010, 1 in 2012. 2 in 2013 and 1 in 2014).
I'm not certain there's much of a case to replace the 717 in the near future. Prior to COVID, reports were that DL -- which had been buying the aircraft off-lease and was expected to make cabin & cockpit upgrades -- had decided to move away from the type, largely due to swelling labor costs. If business travel is slow to bounce back, and the low fare environment persists (as many including WN project), it'd probably make more sense to keep the CRJ around a bit longer.
Given the low marginal costs at Boeing for an extra three or four MAXs a month, I expect that something could be an order for 100 MAX 8s and 100 options -- but I'm not ready to bet on it yet.
I'll bet that when UA announces its pending mega order (some of which will reiterate/replace existing orders), this forum will lose its cool and insist that unless DL buys 787 and the MAX now, it's headed for financial ruin. Because the average person can tell the difference between a refurbished 320 and 320 NEO, you know. (And UA's order appears to be in lieu of its previous plan to renovate its existing fleet.)