Apparently Mussolini and Hitler decided sometime during 1939 that Yugoslavia has no future in the new Europe; that it should be vassalized + reduced in size at the very least, and probably outright dismembered.
But Hitler didn't want to blow up the region and divert attention and resources from his main goals and fronts, so Yugoslavia's fate (be it violent invasion or some kind of Third Vienna Award) was indefinitely postponed. Instead they courted and diplomatically pressured Yugoslavia, and even let some not-so-secret acts of support for the Allies go unpunished - until the March Coup, which made Hitler snap and order the attack.
Italy was toying with the idea of invading Yugoslavia in 1940 (some also wanted to invade Yugoslavia + Greece at the same time), but in the end Mussolini followed Hitler's line and didn't risk it.
So it's certainly possible, and the idea was already there. Not sure what the most plausible way to make it happen is, though.