Please embed a high-quality jpg/png-thumbnail (ideally 500 pixel max wide or hight by images bigger than 500 pixels)  into our xcf files. 

benefit: this would help preview applications to display the contents of less known formats, like the xcf format is, without the need to open the file with the Gimp.
On Mac os, we use Quicklook to preview files in a fast way to flip your files, instead to read only a filename. (i have lots of Gimp files around) 
Lisanet bundles her Gimp package already with a Quicklook plugin, but its -unfortunately- useless slow, and… previews of that plugin arent stored temporarily in the cache. 
I dont know if this enhancement is stricktly Mac-Os related, but i guess that other platforms use similar mechanisms