We already have the widely-supported direction=* tag for limiting the scope of highway=traffic_signals and highway=stop. The commonly used values of this tag certainly have their limitations (ie, there's no clear way, on an intersection, to indicate a "backward" direction for one road and a "forward" direction for another) but I don't think adding an additional stop=*/traffic_signals=* tag will make this situation any clearer.

For situations where traffic control at an intersection can't be properly described with direction=*, moving the stop (or signal) nodes to the approach ways is the only currently viable solution IMO. The way I see it, in fact, tagging the stop on the intersection itself is a "shortcut" that should only be used in unambiguous situations (either all-way stops or those that can be properly described with a single direction=* tag.)

I can see the appeal of relations, because they would allow the intersection to be described in great detail. But they're harder to tag and maintain and I don't think they're worth the complication.

Personally I would also put highway=stop node where a stop line should exist (relations aren't something I would fancy here). What I'm suggesting is to show whether the stop is all-way or minor road only to users. Tagging it explicitly by stop=* would save some trouble and provide more information easily.

The junction=* part is primary grounded in signalized junctions (evaluating number of traffic lights, identifying controlled points to replace highway=traffic_signals at intersection nodes, etc). Either way concerning the former, priority junctions deserves to be tagged even more to caution the uninterrupted major road of priority on mergings (as opposed to zipper merge, or lane adds that needs more detailed/micro lanes tagging).

Truth is I can't quite see the need for the junction tag and exactly how it would function, and these comment boxes, with their limited functionality, probably aren't the best place to clarify it. Using the wiki talk page is also a little awkward. Maybe try discussing on the tagging mailing list and/or writing up a diary entry on osm.org with example pictures for the various situations you're trying to cover.

One thing I believe strongly, though, is that there shouldn't be a highway=stop node on an intersection if not all roads passing through that intersection have to stop there. I consider this "troll tagging", ie, what the main tag gives the subtag stop=minor takes away, which is a problem for any data consumers that aren't specifically looking for that tag. In a sense we already have troll tagging on highway=stop nodes via the direction=* tag, but that's in common use and people know to look for it. Attempting to use an additional troll tag with possibly contradictory information will be problematic.

If this scenario ("this road doesn't have to stop at this intersection but the other road does") is important enough that it needs to be explicitly tagged on the intersection itself (rather than be deduced from the tagging on the other road -- a data consumer can always follow the highway graph and find all this information from the connections if the stop node is there on the minor road) then a different tag would be needed. You might want to propose something like highway=minor_stop that will not interfere with existing software designed around the existing tagging scheme.

I'm still new to OSM so I don't think it's best to be too formal and certain about my opinion yet. I saw stop=* being documented on wiki. I agree with highway=stop at minor stop intersection being essentially wrong, that's why I thought about junction=* as a better option for initial work and other cases. That being said, such incorrect tagging still needs to be detected and fixed.

I forgot to mention one reason is I encountered some priority junction having lane-based restriction on merging, prompting me to look at something that needs to support more complicated schemes like stop:lanes=no|all. Another potential use (I still need to look at international examples) is on roundabouts without physically segregated slip lanes (eg give_way:lanes=no|minor|minor).

I'm trying to see if there's a way to make use of existing tags the most (with backward comparability of course) as I don't want to propose new keys and values like highway=minor_stop unnecessarily.