MyRocks storage engine itself is identical in MariaDB 10.2 and MariaDB 10.3.

MariaDB 10.3 has a feature that should be interesting for MyRocks users. It is the gtid_pos_auto_engines option (MDEV-12179). This is a performance feature for replication slaves that use multiple transactional storage engines.

For further information, see mysql.gtid_slave_pos table.