Since 'Content translation' module is being obsoleted in Drupal 8, the idea is implementing here any missing feature for Drupal 8.
We'll start with some 'Translation set' entity type and then we'll see...
Warning: This project has been replaced by Entity translation. Entity translation works with the Translation module in Drupal core instead of replacing it.
The attempt to transparently replace translation.module with an enhanced implementation failed big time, led to a whole bunch of unresolvable conflicts, and is not recommended. Read #902760: Change module name so it can work with Core translation for details.