If geo-replication master goes defunct, applications running on it should be possible to migrate over ("failover") to slave.
When master comes back, in case of a data loss (either local data corruption or getting to a stale state due to not being updated in the outage window) it should be possible to migrate (approximate) slave state and applications back to master ("failback").
Because customers need it oh so bad.
Result (if any):
It works, but not automated. A skilled administrator is needed to stage and supervise the fo/fb process, and occasionally, make decisions about best approach.