Currently, top hosts widget is tied to host level tags:

It could be improvement to user usability to allow a template level tags to be used in "Top hosts" widget. For example:

db: mysql
env: production

A quite logical use case.

I believe the biggest struggle to bring this functionality to light is "nested templates".
It could be doable to extract the "tags" from the first level template tag. "attach" the template tag to items. Challenge comes to support multiple levels, as each child template can have its own tag.