In order to be able to have better arbitration capabilites for galera, it would be great to have an actual segregation between quorum membership and full membership.
This way garbd would be able to perform arbitration from anywhere without inserting its own lag into the replication process.
That would also allow to developp HA clusters of arbitration which would greatly help with topologie design.
An "easy" way (ops wise) to add it would be to add a parameter to garbd like --membership=full/quorum
membership being full by default and overridden for mysql/mariadb instances.