Status update, the way I see it (i.e. this may be wrong):
@heckflosse, @agriggio and @floessie have been busy cleaning up the RawTherapee engine codebase, removing as many warnings as possible, to pave the way for Stage 2.
Stage 2 is a refactoring of the RawTherapee engine. Currently an image flows through several parallel pipelines - one for the thumbnails, one for the preview, one for the saved image, and one just in case. This has been a problem for years, and the mission now is to consolidate them into one smart, easier-to-manage pipeline. SmartLine, if you will.
Either as part of Stage 2, or directly following it as Stage 3, @jdc’s Local Lab tool will undergo surgery, and an organ transplant will plug it into the new pipeline which will also allow, with @Hombre’s help, to implement a better user interface, hopefully with on-canvas editing.