TokuDB is now open source.
While the MariaDB makers are working on integrating Apache's Cassandra engine, a completely new "NewSQL" engine, promising to bring together the best of SQL and NoSQL worlds, TokuDB is right now a working alternative that is said to bring impressive performance boosts, especially for very large DBs. It is also said to be fully compatible with MySQL, whatever that means, and therefore a drop-in alternative to InnoDB.
While support for TokuDB could certainly live in contrib, Core might be interested in shipping with major performance improvements. We could evaluate if TokuDB would make a considerable difference, and make sure it works with our current MySQL/InnoDB implementation, so a contrib plug-in wouldn't be needed at all. If performance gains aren't impressive or there are major problems or incompatibilities, we would switch over to a contrib solution or won't fix it.