Current Category

That wouldn’t really make sense to integrate into the current widget. Your request would only apply on category pages – so having a checkbox in the widget options list would be confusing as it would appear to do nothing most of the time.

To add to this confusion, what happens when you’re on a post page? If the post is only in one category, it could show subcategories. But what if the post is in multiple categories?

Options like this tend to be more trouble than they’re worth. You might want to build your own specialised plugin to do what you’re after.