More practical (less off-putting) IMO might be 2-level review when an idea/design, API, first-cut implementation and readable/sensible documentation are presented for evaluation. If that's rejected outright, then it saves the author a lot of effort that he might direct onto improving his original design/offering. If the initial concept is accepted, then the author would have a real incentive to keep working and improving his original submission behind more/less stable and already-approved API.
This looks like a pre-proposal often done in academia (and other realms) when preparing a full proposal is a serious amount of work. That said, it would likely increase the number of review managers required as one has to manage the pre-reviews now. - Rhys