Shameless self-plug: If you are looking for an open-source scale-out MySQL that supports both multiple writes and multiple reads, feel free to give TiDB (https://github.com/pingcap/tidb) a try.

Galera is another multi master solution that sits on top of MySQL or MariaDB.

Wouldn't matter. Aurora's value-add comes from its storage nodes. They would have to open source that too.