harbinger doesn't carry a negative sense. It's the nouns it modifies that makes it so (e.g., the harbinger of absenteeism, depression;a harbinger of impending doom!) or the reader already has a negative sense of the topic.
Consider these contexts,
... an awareness of and an ability to work with ambiguity is to my mind one of the harbingers of higher intelligence ...
They crushed Netscape - the harbinger of browsers.
Second, the warbler, another harbinger of spring, sings among the boughs of plum ...
antonym: follow, follower