It would be useful if the WAL protobuf and WALKey class included an optional map of extended key/value attributes that downstream coprocessors could use to annotate WAL Entries. While standard HBase replication would not use them, custom replication endpoints could use the data to make filtering decisions or take actions.
An example use case would be allowing a tool like Phoenix to annotate WAL.Entries to indicate that a given Entry is associated with a particular Phoenix view rather than the base Phoenix table. (Multiple logical views in Phoenix can map to the same physical HBase table.) A custom replication endpoint might choose to replicate some views but not others.