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[[mw:Extension:RelatedSites]], [[mw:Extension:RelatedArticles]] and [[mw:Extension:Insider]] all perform largely the same function - they take a user-supplied link from the article wiki text and stuff it into the sidebar. They differ in how the wiki link is labelled and which section the link is placed on the sidebar:

[[mw:Extension:Insider]] is used by [[voy:template:hasDocent]] to add user page links to a sidebar section named "Destination Docents", the displayed text is the target username without the namespace - see [[voy:Buffalo]] as an example. Multiple links are permitted. An "About docents" link at the end of the list points to [[voy:Wikivoyage:What is a docent?]]

[[mw:Extension:RelatedArticles]] is used by [[voy:Template:Related]] to add article page links to a sidebar section named "Related pages". The displayed text is the link target; namespace prefixes are not hidden or removed. Multiple links are permitted. There is no added "About..." link. These are used to make links like [[voy:Across Canada by train]] (itinerary) to "Rail travel in Canada" (a related travel topic).

[[mw:Extension:RelatedSites]] is used by the wikipedia: and dmoz: interwiki links. It adds interwiki links to  a sidebar section named "Related sites". The displayed text is the name of the targeted site (usually "Wikipedia") without the name of the target page. Multiple links to the same site are not permitted - almost as if a mistaken assumption had been made that there is a 1:1 correspondence between Wikipedia and Wikivoyage articles. If one Wikivoyage topic (usually a city or region) corresponds to multiple WP topics, this breaks. RelatedSites is not a tag function. The list of interwikis to be pushed to the sidebar is in a LocalSettings.php - like site settings file and therefore not configurable without control of the servers. The names of the target sites are in $wgExtraLanguages - an ugly kludge as "Wikipedia" appears among options for user interface language in [[voy:special:preferences]]. Linking [[wikipedia: as if it were a language to be relegated to the sidebar requires that an inline link be [[w: or [[:wikipedia:

Of the three, RelatedArticles looks like the most viable candidate to be extended to replace the other two. It's currently a tag function, <includeonly>{{#related:{{{1}}}}}</includeonly>. Its one parameter is the target page name; it assumed the display name matches the link target (with prefix displayed) and assumes the link goes in the "Related pages" section on the sidebar.

What if these restrictions were removed by adding two optional paramaters, ie:
{{#related: w:Paris, France | Related sites | Wikipedia }} to tell it to link [[Paris, France]] in the "Related sites" section labelled as "Wikipedia"? That would render the kludgy RelatedSites extension redundant. Similarly, {{#related: user:A. Wikitraveller | Destination Docents | A. Wikitraveller }} would replace the Insider extension. There would need to be duplicate link checking to ensure {{#related: Wikivoyage:What is a docent? | Destination Docents | About docents }} only gets linked once, but by allowing the section and link text to be specified in the #related: tag, one extension could do the work of three without hard-coding sidebar section names and with more flexibility.