It's seems like a good idea to me; can we have it, please? With an option similar to "Only show indent line on active node element", but working independently (i.e. indent lines for all nodes & indent blocks for active one.)
Alium tibi quaere fratrem; hic, quem tuum putas, meus est. Titus Flāvius Caesar Vespasiānus Augustus
Maybe this should be independent of indent lines settings and always available as an option. Maybe named something like "structural highlighting"...
Anyway, I made an attempt at implementing it and this is the progress so far. I haven't added any options yet, and there are situations where it could look better (screenshot 2).