If you're not sure about the viability of an idea or a proposal, you might refer to it as an "iffy proposition." However, there are several other synonyms that can be used to describe such a situation. For example, you might describe it as a dubious proposition, a questionable proposal, or a risky idea. Alternatively, you might say that it's a gamble, a leap of faith, or a shot in the dark. No matter how you choose to describe it, an iffy proposition often requires careful consideration and weighing of the potential risks and rewards before deciding whether or not to go ahead with it.