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: https://hive.blog/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.
If we try to check with a normal post we can have 0 feedback, I leave in a completely different world here in Hive.
If we try with normal post this can be read by 10 people and that tell us nothing about our idea.
If we enter a proposal it is read by many people, because there are very few proposals in the system, so we have a feedback that has some value.
But, what about our system?
One of the main points of creating this new Hive Improvement community was to allow ideas to be proposed and get attention without needing to make a proposal first. In other words, you can get visibility somewhat similar to the extra visibility of the "proposals" page, without having to pay the proposal fee.
The advantage of this method is you still get some attention and feedback for your ideas (as opposed to a random post, where you don't think anyone will see it, apparently), but the blockchain doesn't have to do the special processing for voting on it.
As for your timing idea itself, I don't think it should be fixed to 30 days. Of the ideas I've heard suggested so far, several seem workable , I like "10 HBD for up to 60 days, +1 HBD for each day thereafter" best.
Sorry, can be I'm not able to explain my idea.
I try again.
The idea is that you can ask for funds as you do now, but you have only 30 or 60 days to collect the necessary votes, if you don't, your proposal is deleted.
You don't have to pay more for a long term proposal, everyone as a limited time to successfully have an entry level support.
Let me know if is not clear again.