The phrase "waiting to happen" implies that something is just on the cusp of occurring. There are various synonyms for this term that can convey the same sentiment with a different tone. "Imminent" indicates that something is very likely to happen in the near future, while "impending" suggests that the event is looming ahead. "Inevitable" conveys a sense of inevitability, as if the outcome is unavoidable. "Brewing" suggests that something is gradually building towards a climax, and "simmering" implies a slow but steady buildup of tension. Using these synonyms can add texture and nuance to one's language, making for more expressive and impactful communication.