I believe DL will return to service most of the 717s. Aside from the A220, the 717 is cheapest to fly mostly empty.
After that, it depends on how travel comes back. If international travel is anemic, DL will be flying mostly 767s. Again because they are cheapest to fly empty. The 77Ls will fly for cargo and there are only 10 of them. I would expect after that the A359s and A339s will be the rest until travel goes over 60%.
There is also currency. 90 days of no flying, and a pilot needs to requalify. Some seemingly suboptimal flying will occur in order to keep a subset of pilots current. Beyond that, who knows?
DL will be reducing its anemic schedule to largely a skeleton soon. It's logistically impossible to fly this schedule with primarily 717 or any other fleet type, even if DL wanted to; fleet types will be diverse, and DL will continue to market "social distancing" to make people feel
safer. It's unlikely additional flights will be added until traffic warrants... put simply, DL isn't going to fly more empty planes -- and even a 60% LF would be considered "empty" -- than it has to.
Alas, I wouldn't be surprised to see some of the 717 returned to Boeing. DL intended to retire these planes by the end of the decade, anyway, and said they would give the airline "flex capacity" in later years. At this point, there's little reason to keep the entire fleet around... the money saved on maintenance (including possible storage), lease payments, etc. would be better spent elsewhere - might as well add more 220 when they're necessary. The primary reason ATL and CLT can handle so many mainline aircraft is because of their proximity to vacation points south (mostly Florida), and the large number of people headed there. It's going to take years for this traffic to recover -- numerous smaller markets won't get mainline back for quite awhile. The fleet is going to shrink.
Similar reasons would apply as to why DL would outright retire many 767 now.
Good because most of the 763ERs are tired.
It's a psychological thing. The 763ER received brand new interiors; the 777ER and legacy 330 did not. Yet so many a.netters feel a late model 763ER is old and tired, whereas they don't feel the same way about the 777ER -- which are older -- or legacy 330, which range from a couple to several years newer. Post-merger, DL replaced the 330 on several long-haul flights I flew from DTW with the 763. I absolutely hated the 763 -- the interior design was old, and the in-flight product was archaic. But today, I love the 763 - easily my favorite ride within the long-haul fleet.