It is dreadful news of course. The top priority, which will be on-going, if it is not already complete, will be to determine the cause(s) of poor Riva's symptoms and her death. Plus continuing close monitoring of the health of Maya, Anjan and Indali. In the longer term, provided the coast is clear, there will have to be some moves of course. The choice will depend on the availability of animals, the capacity of other collections and negotiations with the studbook keeper.
There are several possibilities. Move Anjan and bring in another cow and then a bull - hoping to re-establish a breeding herd. Move Indali and bring in a couple of young bulls to form a bachelor herd eventually. Move both Anjan and Indali and bring in a couple of older cows to become a retirement home. Or go out of elephants completely, at least temporarily: that would allow modification/extension of the elephant house to accomodate a new herd (African or Indian) or for a different species to attract visitors - I can imagine the basic structure of the building adapted for a troop of gorillas.