Hi Jens,
Really excited to have someone pick up the torch.
As far as low-ish hanging features there's a neat branch with some map features:
https://github.com/abrauchli/shotwell/commits/libchamplain which last I tried (2013?) worked pretty well.
My big wishlist one is faster importing, there's a lot of work that later gets re-done during import or the results of which end up being discarded, so it can take quite a while even on a modern machine to import a good number of photos, and frustratingly the cpu utilization during this period isn't even all that high.