Highlight parent when collapsed
I agree with @ldubya. This idea appeals most to me and has been in the back of my mind while using Subform recently.
I scroll through the tree trying to see which box corresponds to the one I have selected on my artboard. I do this often because I want to select its parent -- especially when the children fit snuggly inside their parent or otherwise have trouble clicking on invisible boxes. I find myself looking for an indicator that would tell me which node I need to expand to find my selected box. I'm happy to manually expand nodes down the tree to reach my selected box.
What should be highlighted? Should it have the same or different highlight from the selected box? If the selected box is 4 levels deep and you expand the first node, does that first node need to stay highlighted? I think it should no longer be highlighted. I also think that the highlight/indicator for the node that contains the selection must look different than the highlight for the selection. There should only be one of each highlight/indicator type.
However! Perhaps there is a keyboard command I haven't learned that would be a better solution to my problem: click the child and navigate up or down the tree with a key.
Reveal element in tree
This seems like a good idea to me but slightly less appealing than the highlighted/indicated parent.
I'm neutral about this idea.
Collapse/expand all descendents
I don't think I'd use this much.
Auto-scrolling tree to selected box
Despite suggesting this myself, I now believe I'd dislike this feature. I prefer having control over the view of the tree, even if I have to manually adjust it.
Auto-expand/collapse tree based on artboard selection
Again, I prefer having control of the tree view. To be clear, the auto-expand would happen when you select a box that is inside a collapsed node and auto-collapse would only happen when you select elsewhere AND the node had been auto-expanded, it wasn't manually expanded. I can imagine auto-expand/collapse being equally nice and annoying, depending on what you're trying to do. Nice when you want to keep the tree tidy, annoying when you were expecting to see a node open and it isn't anymore -- thwarting your brain's attempts to be efficient thru pattern recognition.
Auto-expand for drag'n'drop structural rearrangements
That might be nice. The delay is important. Does the node collapse after you move your cursor away? Is there anything that would cause the tree to jump around based on this behavior? It can be hard to get this interaction right. If you don't have a delay, then a node expands and you have to scroll down to get to your target. If you scroll past an expanded node, does the next node suddenly jump up above your cursor because its sibling closed?