Sorry I just realised that you ask this feature for JIRA Cloud, nFeed is only available on JIRA Server.

My apologies.

My comment was: 

Hello,

You can use nFeed - an add-on developed by Valiantys Software - to create JQL-feeded custom fields.

There is a step by step tutorials in the documentation that shows how to configure such a field: https://valiantys.atlassian.net/wiki/spaces/NFEED511X/pages/93311056/Quick+start+how+to+create+an+issue+selector

Hope it can help.

Christophe
Product manager @ Valiantys

if we don't want to use JIRA simply for Task Management (ticketing system) and IT Support, but also for enabling various processes within a corporation (for enterprise process management, something like SAP, but 10 times better!). JIRA is powerful and dynamic enough for this. Only a couple of small features would contribute a lot. JQL-based custom fields are a very important feature on this matter.
Example 1: Using JIRA as the central repository for server, database and applications information within the IT landscape
> In this case, requests for a new server, database or application is done via JIRA. So far it's good and we don't need any extra features.
Now, if we want to have a clean system with reliable data, we don't want to have a free-text field for the IP address of the server, when we are requesting a new database. We want to be able to choose the target server for a database based on the existing (requested, approved and installed) servers within the landscape. This can be done via a JQL custom field. It means, I define a select-list custom field and configure it in a way that it's value would be determined based on the active servers in the system. The JQL looks something like this:
Project = "Server Installation" AND issuetype = Server AND status = Installed
Then I configure the value of what field (in the resulting issues) should be shown as the available options; and the value of what field (in the resulting issues) should be saved as the value for that custom field.
Imagine that servers-info is saved in an issue with fields including IP-Address, VM-Name, Cluster, etc. I should be able to define: Show VM-Names as the option-names of the resulting select list of the custom-field "Target Server", and use IP-Address (or Issue-Key, or whatever else) as the value that will be saved for that custom-field.

Example 2: Using JIRA for handling and managing invoices and budgets.
If we use JIRA to manage budgets and invoice-payments, we have two projects (Budget Management) and (Invoice Management). What we want is to be able to select an existing budget for an invoice in the "Invoice Management" project, when we want to pay that invoice. The field for this "Allocated Budget" should be pre-populated based on the existing approved and active budgets (that is done obviously using a JIRA workflow in Budget Management project). So we use this JQL custom field to define the "Allocated Budget" field based on a JQL like this:
Project = "Budget Management" AND issuetype = Budget AND status = Active

The same configuration mentioned in Example 1 should be provided here as well.

As you see, we are* building rational relations* between issues by using this type of custom field.
I hope I could convince you that this makes sense!