Windows 10 build 15064 for PC and mobile build 10.0.15064.0 is now under testing internally. The new Windows 10 Creators Update Insider build 15064 is expected to be the next Insider build for the Fast Ring members.
Recently, Microsoft also released Windows 10 Insider Preview build 15063 for PC and mobile build 10.0.15063. The build is currently available for Fast Ring insiders. The latest insider build brings bug fixes and improvements.