Nested categories in Advanced Search Form

Dear Support Team,
for every custom post type, I can define categories. I can also define subcategories. Let’s take as an example “Guitar lessons” as category, and “Search guitar lessons” and “Give guitar lessons” as subcategories.
Currently, in the “Customize my search” form, all three categories above would be listed on one level. Ignoring the sub categories.
Is it possible to display the sub-catgories in a nested way? That would look much more clean and give more orientation.

Really smart would be, if results in the “Popular Category” widget would also calculate the results concidering the sub-categories. So if there were 3 results for “Give guitar lessons” and 1 for “Search guitar lessons”, the number in the categoriy above, “Guitar lessons” should be 4. Plus possible results that have just the category “Guitar lessons” (… doesn’t fit into my example well, but imagine my example would have music styles as sub categories, and somebody does not enter these sub-categories, just the one above.)

Yes, this nested way of displaying needs some more thoughts (… for example, if a post is in a sub-category, plus the one above, it must not be counted twice). But it would be much more usable.

Thanks,
Tom