> Ability to send messages to slack on successful / failed builds

That should be easy enough, request a new cloud builder Slack. Currently this is possible though using the curl cloud builder and constructing a custom curl request to the Slack API.

> No story on how to store secrets

Agree, need a way to mark variables as sensitive/secure. Don't show them in the build history in plain text and make the input field of password type. Encrypt somehow when stored as well.

> Conditional build steps AT ALL

Agree, this is a biggest missing piece.