The goal of this would be to force the users to put issues in a hierarchy rather than mandating that every issue is important.
It seems that your are looking to define ranking more than defining a issue hieararchy.
what you can do is to create a rank numeric custom field (global context) and to use it to define the rank. At that point you could use it to order your filters.
project = XXX order by RANK DESC
If you are planning to install JIRA Agile it provides a great ranking feature usind drag and drop (it uses a rank field as well behind the scenes) https://confluence.atlassian.com/display/AGILE/Ranking+an+Issue
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