While I know that it is probably unlikely that DL will acquire more 764ERs (or any 767 variant), I wouldn't say it is completely out of the question. Either way, if DL were to acquire any new 767s, it would probably be for the 764ER.
We know that the reason that DL's 764ERs all have the "MH" suffix in their registration numbers is due to the fact that the prototype 764ER was registered N825MH by Boeing, and DL chose not to change it, and thus registered the rest of their 764ERs N826MH-N845MH.
So, if DL were to order more 764ERs, how would DL register them? Most of the N8xxMH registrations after N845MH are taken, so that probably wouldn't be an option. Specifically, I wonder if DL would even use the "MH" suffix. In terms of ship numbers, I would guess they would be the 19xx series.