I'd like to use a word like "foreshadow", or a word similar to it, as long as the reader will understand what is to come is going to be good.
Is the a subjective interpretation of "foreshadow" or "omen" a good thing or a bad thing?
What alternative would mean something positive?
If you want a word with a similar meaning to foreshadow, but with the connotation of a good outcome, you could use the word augur. Whether it's appropriate or not depends on the exact use you want it for, but the word augur is indeed generally associated with something good. Consider this Ngram:
Among words of its kind, foreshadow is one of the most neutral. Its synonyms forebode, threaten connote problems, but foreshadow itself seems to not do so: "To presage, or suggest something in advance."
I usually think of presage as neutral, i.e. of its "An intuition of a future event; a presentiment" meaning, but I see it also has a usage that may be negative: "A warning of a future event; an omen".
The words I meant by "words of its kind", above, are synonyms (not all relevant) of presage, foretell, and omen: announce, annunciate, anticipate, augur, augury, bode, call, forebode, foreboding, forecast, forerunner, foretell, foretoken, harbinger, herald, indication, omen, portend, portent, predict, premonition, presage, prognostic, prognosticate, promise, prophesy, sign, threat, threaten, warning.
In my experience, foreshadow is a relatively benign term. I don't expect malevolence to come from its usage. It's a simple forecasting of an event that has not yet occurred. ("In retrospect, Ellen's habit of playing teacher as a young girl foreshadowed her career as an English lit professor.")
Omen or portent, for that matter, usually has a connotation of ill-fortune soon to befall someone. ("The black cat crossing one's path is seen as an omen to the superstitious.")
While foreshadow and omen can apply equally to good or bad situations per their dictionary definitions, I think there's a minor tendency for both to be negative in common usage. (I think OP has probably seen that too, hence the question.) Look at the contexts in which they're often used; we talk about a movie foreshadowing something (cue ominous music!), usually the turn of events that's going to make things hard for the heroes, or getting a bad feeling when you receive an omen. Not always and not a super-majority of the time, but there's a tendency in my experience (American English).
A neutral word for "providing an indication of something to come" is telegraph. This takes an object: "telegraph a move", "telegraph an aquisition", "carelessly telegraph the 'surprise' party", etc.
One can also say a harbinger or talisman of good luck or fortune. These can figuratively be used to refer to any event.
