hholzgra, to a beyond new global var alternative, like yours

> maybe also an explicit MASTER_SSL_USE_DEFAULTS option to CHANGE MASTER

how would look to you the following syntax form

CHANGE MASTER 'new_connection' TO LIKE 'default_connection', ..., any-option = custom-value

so the default configuration for a new connection 'new_connection' would be first copied from 'default_connection' "parent" and
any option like any-option value's can be overridden/customized?

That might work, too. Advantage would be that no additional config settings are needed. Disadvantage may be that some individual settings might get inadvertently, e.g. we might want to exclude log file and position from being copied ...

How about having all the options to CHANGE MASTER also as user variables set in my.cnf (with a MASTER_ prefix for all options).
Then one could use CHANGE_MASTER .. ANY_OPTION=DEFAULT to use the value from the corresponding variable?

combining this with LIKE 'default_connection' would be powerful. The problem with default_connection is that in this case we need a way to disable some options that should not be set (like disabling SSL).

it doesn't make much sense to have defaults for all options, in fact it'll be more error prone to do that. The list of options (according to KB) is

    MASTER_BIND = 'interface_name'

  | MASTER_HOST = 'host_name'

  | MASTER_USER = 'user_name'

  | MASTER_PASSWORD = 'password'

  | MASTER_PORT = port_num

  | MASTER_CONNECT_RETRY = interval

  | MASTER_HEARTBEAT_PERIOD = interval

  | MASTER_LOG_FILE = 'master_log_name'

  | MASTER_LOG_POS = master_log_pos

  | RELAY_LOG_FILE = 'relay_log_name'

  | RELAY_LOG_POS = relay_log_pos

  | MASTER_DELAY = interval

  | MASTER_SSL = {0|1}

  | MASTER_SSL_CA = 'ca_file_name'

  | MASTER_SSL_CAPATH = 'ca_directory_name'

  | MASTER_SSL_CERT = 'cert_file_name'

  | MASTER_SSL_CRL = 'crl_file_name'

  | MASTER_SSL_CRLPATH = 'crl_directory_name'

  | MASTER_SSL_KEY = 'key_file_name'

  | MASTER_SSL_CIPHER = 'cipher_list'

  | MASTER_SSL_VERIFY_SERVER_CERT = {0|1}

  | MASTER_USE_GTID = {current_pos|slave_pos|no}

  | MASTER_DEMOTE_TO_SLAVE = bool

  | IGNORE_SERVER_IDS = (server_id_list)

  | DO_DOMAIN_IDS = ([N,..])

  | IGNORE_DOMAIN_IDS = ([N,..])

Out of these, MASTER_CONNECT_RETRY, MASTER_HEARTBEAT_PERIOD, MASTER_SSL*, MASTER_USE_GTID look like one might want to have the same value for them over all masters in a multi-master replication. So they would benefit from global server-wide defaults.