I would vote for a second snapd instance which doesn’t load/provide anything of what the system instance does. It will load a different mode which provides for now just the dbus endpoint snapd-xdg-open provided before. Over time we can grow that in different other things we may want to introduce for the user session (like the xdg-desktop portals @sitter proposed here). If the implementation is in C or Go, I don’t really care but giving the nature of the existing snapd code base Go sounds the way to go.