In a world of infinite resources, once the WMF decided that a specific proposal was a good idea, it would move that concept into the planning stage, and continue on through to Release and Maintenance. But, of course, we don't live in that world. So, the question: what are the factors that go into deciding what moves forward, and what is deferred?
Related to the question of who decides is estimating the cost of implementation. If two proposals have about the same level of expected benefits, but one will cost two or three times more to implement, then the WMF should probably implement the lower-cost proposal, and possibly defer the higher-cost proposal. (Note: the Concept phase now includes "technical debt", which is the eventual consequences of any system design, software architecture or software development within a codebase. That's not at all the same thing as cost: a proposal might require a large number of staff-hours to implement, and yet be relatively easy to maintain, and affect few other parts of MediaWiki code.
In short, if the WMF truly wants to be transparent, there should be a visible decision process. That doesn't mean that the community has to vote (or !vote) on most (or even any) products being developed, but it does mean that decision-making should be more than a group of developers saying "Okay, what do we want to work on next?", with regard to feature development.
What might this look like, on the chart? Perhaps a Prioritize phase:
- List of benefits
- Estimate of costs
- Synergies with other product development
- Setting priority: Very high to very low
Obviously, "benefits" typically would not be described in dollar terms (WMF is not a business). But WMF should make clear who it sees as benefiting: novice editors, editors using specialized tools, wikignomes, prospective editors, etc. That in turn encourages a discussion about the relative merits of different proposed products.