Is this in a software context? If so, mandating structured RFCs will help a lot. It will channel random streams of thoughts into constructive, actionable proposals.
Have your first RFC be about how to structure an RFC. Make a cost/benefit analysis (in real money if possible) be a mandatory part of the proposal. Commit all of them to a main branch in git even if they are rejected because you would preserve the original discussions around that particular proposal.
Basically anything that can be an epic ticket can and should be an RFC first.