It's great to see that getting to an open source foundation is a top priority! As a member of the CNCF's technical oversight committee (and as someone who personally loves RethinkDB), I would love to champion RethinkDB for the CNCF -- but it matters much more to me that it gets to an open source foundation than it is that it lands at the CNCF in particular. RethinkDB team: please reach out to me or to any other CNCF TOC member if we can help!
From an outsider's perspective, CNCF seems like it would be a great fit for a project like Rethinkdb: it looks to have the forward momentum Rethink needs right now, and the proximity to other high profile projects like Kubernetes could encourage adoption.