I love the power of additive/subtractive Boolean modeling where the full history is always maintained. This makes Tinkercad—and by extension, me ;)—very powerful, because I can always go back and modify the size of a shape or hole used in a group and then re-group it.
When many groups overlap in 3D space, ungrouping one in order to make a modification results in many overlapping shapes. This makes it hard to re-group the shapes, because it's hard to select all the right pieces without accidentally picking up unwanted pieces.
The feature that makes the problem not so bad is the (delightful) ability to double-click to "enter" a group, modify an object, and then leave the group. In this way, I don't have to worry about re-selecting all the pieces that made the group in order to re-group. Lovely!
However, my workflow tends to produce groups several levels deep. If I want to modify a shape more than one level deep, I must ungroup. To avoid the problem of finding the correct shapes when I re-group, I have to either Hide all other shapes that aren't part of the groups as I dive down (which only works for a one-level dive, since I can't Unhide in groups), or I have to move the group far away from the rest of the model before I ungroup (moving to a new location at every level).
My feature request is to find a way to dive into nested groups without actually ungrouping. Here are some ideas for interfaces that might allow this:
Sole Group Edit Mode
Alt-double-clicking on a group hides (or heavily ghosts and locks) all objects not in the group, and adds a breadcrumb (similar to NVIDIA DRIVE Design, or Adobe Illustrator) allowing the modeler to back out one level at a time, or all the way to the top.
This requires a new 'breadcrumb' UI, which requires some way of naming or otherwise identifying each group. Perhaps quick-n-dirty thumbnails of the group at each level in the breadcrumb, instead of a name? To make them more clear
Group Hierarchy Tree
A new 'panel' can be shown that is the full hierarchy of groups in the scene (similar to Adobe Illustrator, or any outlining program). Shapes not normally shown in the scene (e.g. a hole that has already been grouped) are listed in this graph. Hovering the mouse over any item makes it appear in the 3D render. The item can be selected, and its Inspector shown, or handles edited, with all changes properly propagating up the group.
Names of items could default to just be the name of the primitive (or "Group"), perhaps suffixed with "Shape" or "Hole". However, the modeler would be allowed to edit names in this palette; this would make it easier to find a key piece in the future without hovering each item.