I agree that Shotwell is a bit unexciting. But take a close look at digiKam, which has many of the features you mention, including Light Table and diff view. I use this as the primary app for my workflow, with GIMP for heavyweight image manipulation. Between these two I get pretty much all the functionality I need, although I also use Hugin for panoramas and Luminance HDR when necessary.
Good luck with the development. I’ll certainly give it a go and keep an eye on progress.