It could be useful to have an “Excerpted From” RG-RG relationship. Sort of the inverse of “Included In”, but “Included In” implies that the included RG is in more or less the same form as its standalone version.
I’m thinking of cases where there’s a multi-disc set, and then a single-disc selection of tracks from that set. For example, Early Ellington and The Best of Early Ellington. All the tracks on the “Best of” release also appear on the larger set, but interspersed among the other tracks.
I guess it’s kinda-sorta the same discussion as Highlights releases: same or different RG? - which was not super conclusive. I do still feel these should be their own RGs and in that case they should have their own relationship to match them as you suggest, but not everyone seems to agree 
my thought is if there’s a relationship that can be added between the two, I see no issue with seperate release groups, in the case of the above, highlights, and album samplers
Separate RGs is the way to go, I think. When the “Best of” release has only a third or a quarter of the tracks of the full set, it doesn’t make sense to consider them the same.
Linking to the ticket discussed in the thread @reosarevok cited above: [STYLE-1241] New release-release relationship: Highlights - MetaBrainz JIRA