I know it is not a replacement for Aperture, but at least you can access some of the familiar raw conversion features in Aperture with the new application from Nik Bhatt (Gentlemen Coders) - he was the Senior Director of Engineering for Aperture at Apple. He has developed a vary basic raw converter called "Raw Power" which will look familiar to those who use Aperture. It is a nice implementation of raw conversion using the Apple Raw Core and can be run as a Photos extension or a stand-alone application.
Right now, it appears to be very basic and lacks many features that a full-featured raw conversion application would have (batch processing, conversion color space choice, various local adjustment tools, etc.). However, if all you want is access to the good old days of Aperture's raw conversion engine, this may be the ticket for you. And, at $9.99 on the App Store, it is a no brainer.http://gentlemencoders.com
Here is an interview with the developer and a walkthrough of the application:https://youtu.be/BSpHIT0ok_Q