If my proposal ended today, could I create it again the next day?
Unfortunately, subjects re-posted with the same claims of vague or unsubstantiated support are no longer supported by this process. Area 51 is not the place to find an audience. When you submit a proposal for a new site, you are implicitly agreeing to the instructions in that form which state:
When you submit a proposal for a site, it is generally assumed you have access to an audience to build it. Area 51 cannot find that community for you.
You are asked directly: "Please describe or link to the community where you will be publicizing this proposal:" even though many authors hand-wave that requirement with responses like:
"people interested in [proposal title]"(← yes, these are real responses I receive)
I understand some might not have a specific link ready, or only now realize they are responsible for rallying the community they purport will build their site. But when a user simply re-submits the same proposal with no indication they have the means to actually build a site, I have to close it as a duplicate (failed) effort.
Yes, you could theoretically propose it serveral times without breaking any rules. Though it's better to gain a lot of people to your cause, then create the proposal for them to join. That way, your proposal is not deleted for not having enough followers.