The DC9 replacement process, already started by NW (CR9's and Embraers), is likely to continue and maybe (hopefully) extend to the MD fleet.
The A319s, A320s and older 737s are a different story. Not sure where that is going. DL has been an "all Boeing" airline traditionally, so I doubt they'll be looking at Airbus, but you never know.
I'm just looking forward to DL retiring the 767s (which I very much dislike), but that's many years away. Maybe they will be replaced with 787's and DL is just delaying some of them in favor or 777s which may make more immediate sense. (The jumbos are here to stay, but for how long, really?)