Most questions tagged as "reference-request" seem to be people looking for specific papers, or for proofs of specific facts. I would guess that a "roadmap-request" is a request for, well, a road map - which sequence of books or papers one can use to learn some new topic. Of course there's no reason why this isn't a reference request, but in practice there are two very different types of reference requests and it might help to give them different names.

Of course, there's the problem that people won't know what the different tags "mean" if they have meanings that are only apparent to people who read meta.

I think roadmap discussions aren't suited for MO. Especially since they are usually discussions. MO forces its interface onto the answers, so even when they are answered by professionals, usually the next answer coalesces the former. We get these fibonnaci threads... Wouldn't it be better if there was an actual site for these roadmaps? Actual diagrams with nodes of points of view, leafs of books, thorns of cutting edge papers.