I would love to see a simple feature added whereby every single widget that's added can optionally have a condition applied to it so that the widget can appear in some places and not others.
Some widgets make sense on home page, archive, single posts, pages, but not everywhere. Some people might want a logo to appear within a set of child pages belonging to one particular parent page, but nowhere else on the site.
Naturally, there will be many cases where someone will want a widget everywhere, and that's why the conditionality should be optional -- don't specify any conditionality at all, and it's just like it always was, no extra work for the user, and the widget appears everywhere. But for those who do want the option, adding this simple function would open up a world of customization for widgets without requiring that any number of widgets themselves be further complicated.