I think @silverpill had something like this in mind when they were making FEP-e232, although I don’t think its a requirement. I would also think that it might not be a good idea, since an implementation on the careful side might only want to implement a FEP that is already finalized, creating a chicken-and-egg situation.
Regarding the issue of FEPs just “trundling along” for the most part is I think an issue that some of the bigger implementations (especially the elephant in the room) don’t seem to care too much for interoperability, let alone FEPs, if they can get away with it. I think the elephant in the room in particular might be quite happy to be the one to mostly decide how the W3C recommendation is to be interpreted and the unspecified but necessary parts are to be filled.
Having “only” a few other smaller implementations implement a FEP is kind of cool because of the collaboration, but on the other hand also frustrating if it does not get any wider adoption.