Formal:
* Chance
* Occasion
* Prospect
* Opening
* Advantage
* Likelihood
Informal:
* Break
* Shot
* Go
* Try
* Chance to shine
Other options:
* Window
* Gateway
* Doorway
* Potential
* Scope
The best synonym depends on the specific context of your sentence. For example, "chance" is a good general synonym, while "break" might be better if you're talking about a lucky opportunity.