Unlikely that it will be a current Archbishop and Chaput wouldn't stand for Mahony telling him what to do for a year. Also, considering the diametrically opposed history those two have concerning EWTN and Mother Angelica, naming Chaput as Coadjutor could result in a bloodbath.
Anyway, I think this will be the case, specifically, because of the administrative experience needed, a certain amount of diplomatic knowledge and, to be perfectly honest, knowing how to deal with major donors are going to have to be in place before the successor is named. Whoever it is is stepping into a quagmire of hornets nests and it going to have to have intestinal fortitude that would give the rest of us heartburn.
Practically speaking, a reigning bishop becomes a figurehead as soon as the name of his coadjutor is known. While ceremonies will give deference to the bishop, in practice the actual decision-making is done by the coadjutor, and all those who work in the diocese understand that they have a new leader (the coadjutor) whose official installation as reigning bishop is simply a matter of time.
This appointment by the pope of a codajutor for LA is an extremely significant and strong public act of no confidence in Mahoney, and ensures the history of his episcopacy will remain forever marred by questionable doctrinal and pastoral actions.