A better web slice tool that will possibly slice many shapes (not just rectangles).
Implement the combined selection tool
Improve support for importing and exporting PSD files, including support for reverse engineering of Adobe gradients and brushes.
It's gonna be a cool summer.
This isn't to say these things will come to pass in Gimp-2.10 but i have seen stranger things happen.
I think they should try and get that foreground select tool from the RGGJAN fork working in Gimp-2.10.
And what about that clone tool that was being worked on?