Well, .kra is supposed to be an internal file format. I know that with what happened to .psd nobody cares about such things anymore 
but if we talk about integration (with things that are not just image viewers), we should be talking about something more like .ora, which is supposed to be an interchangeable format. I know .ora is limited for now, but that’s why we need to work on it an extend it further, if it’s needed.
The developers of any application that you ask for support will be much more happy supporting .ora than .kra, too.
For the image viewers or file browsers… I dunno, for example on Cinnamon I just added a file that makes pretty thumbnails for every .kra file, it only needed an addition to configuration. It would be great if it was done by default, but it’s fine too.
tiar@tt ~ $ cat /usr/share/thumbnailers/krita-kra.thumbnailer
[Thumbnailer Entry]
TryExec=unzip
Exec=sh -c "unzip -p %i preview.png > %o"
MimeType=application/x-krita;
tiar@tt ~ $ cat /usr/share/thumbnailers/mypaint-ora.thumbnailer
[Thumbnailer Entry]
TryExec=mypaint-ora-thumbnailer
Exec=mypaint-ora-thumbnailer %i %o %s
MimeType=image/openraster