This suggestion was closed, but the reason did not address the OP's need.
Issue links are not anywhere as useful as the Epic links, nor sub-task links.
Sub-tasks are not available to Customers in Service Desk, so are not an option in that environment.
Epic links are limited to a single issue type.
A workaround would be to offer a means of creating additional Epic-type Issue Types.
The other option would be to introduce more functionality to Issue links so they can be managed from the parent issue as Epic links and Sub-tasks are.
The suggestion that URL Custom fields meets this need is incorrect, as it only works with URLs and not Issue Keys. It also is just a list, which does not allow for management of the issue.
Lastly, Issue links do not provide a single one-to-one relationship of Parent to Child as the other options do. Issue links go two ways essentially, and you can link to multiple other Issues. This creates a web-like relationship as opposed to the Parent-Child relationship needed.
Re-raised: https://jira.atlassian.com/browse/JRASERVER-71109