The DoJ will not be allowing AA/UA/DL to purchase another carrier unless that carrier is on the verge of being disolved in bankruptcy. Period. But even if it were to happen, DL would have to give up too much in order to make it go through (like gates at compacted airports and slots at JFK). There are much cheaper ways for DL to obtain two gates at DAL and some more A320s, if that is their main objective.
File this rumor under A-Nut fantasy.