They've been working on this for like a decade, in a sense. They're rewriting the core as a library, GEGL, which supports non-destructive editing as well as other long-requested features. My impression is GEGL is largely done; the remaining work is to port GIMP to it. Realizing these gains for non-destructive editing is planned for v3.2, which is probably years away still. The other major initiative since forever has been porting to GTK 3.