I think your basic premise here is flawed. You seem to be assuming that you must make a proposal first to check if your idea is good or not. But we already have a good system for vetting ideas before proposals using normal posts, in my opinion.

If your proposal is for some new idea and you're not sure if people will be interested, make a post about it first and see what feedback you get. We even have a specific community now for getting some extra attention for such ideas: /trending/hive-102930. This can be done without spending any money.

If the feedback seems good and you think there's a reasonable chance it will get funded, then you can take the next step and make a formal proposal.