i want to make a custom animation for the plasma "loading" animation (which is what you see in the notifications when downloading a file etc)

i tried to mess up with /usr/share/apps/desktoptheme/default/widgets/busywidget.svgz but i can only make animations that rotate

how can i make any other type of animation ? (like linear motion / color changing still objects)

I would recommend asking on the Plasma mailing list, plasma-devel@kde.org regarding this. You may need to investigate the Plasma javascript API to do this.