New PvE mission coming for the Archives event. Featuring a new map, Rialto, that's set in Venice, Italy. A PvP version of this map is planned to be added to the QP/comp map pool as a payload map, though the PvP version is coming later (planned for PTR testing alongside the upcoming Hanzo changes).
Reaper will also be getting a Soldier:24 skin.
Both the Uprising and Retribution missions will have story and all-heroes modes available, so either the arcade mode will be 80% PvE for the duration of the event or they've done some creative UI work.
Apparently the all-heroes version of Retribution will be more dynamic than Uprising.