If a subsection of your library of assets need to be added to a region on a page, you need to either add a widget per asset or create a new asset with everything in it.
Instead, it would be great to have a new widget which accepts more than one asset. These may or may not be restricted by asset type by the template developer.
If this widget had some sort of dropdown, that would be good too. That way, users could only but X amount of assets into the ‘multi-asset widget container’
Another way to allow this using current functionality is to leave the region empty and allow editors to add multiple widgets at the page level. However, there is no means of regulating what widgets types they add and we can no longer control the type of content that goes in that region.
A new widget that accepts multiple assets will allow you to restrict type and perhaps allow you to restrict the number of total assets allowed as suggested by Alyce.
This functionality is available via the Widget Builder.