After thinking on this for a few days, I think an A321XLR order is likely to replace the 767s. DL has too many TATL routes that need small international aircraft to give up all the 767s without a replacement; the 330Neo is just too much airplane, and the 330-800 is pretty inefficient.

A fleet of XLRs configured with lieflat J would let Delta introduce a quality transcon product competing with Mint and cover a wide variety of ATL/JFK-Europe point to point routes efficiently at reduced traffic levels.

I think it makes more sense than picking up a 787 sub fleet at this point, unless Delta sees an opportunity on the used market they can’t pass up.