Support

No, this is not going to be implemented. Ticket IDs, like all record IDs in Joomla, are integers for very good reasons. 

Switching from unique integer keys to a string key would make things 20 to 50 times slower, making ATS unusable. SEF URLs (URL Router) implementation would be nigh impossible as any string could possibly match. This means that adding the ticket title to URLs is no longer possible, breaking existing installations. Replying to tickets by email would have to be removed, as there's no longer a pattern to match for ticket IDs. Having to somehow figure out a prefix and a unique ID within that prefix space introduces a slew of problems including concurrency (two tickets filed at the same time could get the same ID). The Joomla MVC API is designed to work with integer IDs. Using strings would require rewriting parts of the Joomla API. This means that every minor version of Joomla would need its own ATS version which is unsustainable. Not to mention that it would mean constant introduction of bugs because of small changes in the Joomla API which need to be carried over and tested across a dozen different ATS versions and half a dozen PHP versions. Managing categories would be very challenging. Did you mean to enter the same prefix across categories or not? Did you mean to not have a prefix at all? Changing prefixes would be impossible, especially if you try to change a prefix to a different one that already has tickets in it. This is an undefined operation. 

There are many more issues which would come from this kind of change. But even what I mentioned basically kills ATS as a viable helpdesk component.

I understand your use case, and need to warn you that you are trying to shoehorn a helpdesk component into a role it is NOT designed to fulfil. Helpdesk software is neither an ERP, nor a CRM. You could always create a support staff-only custom field for an RMA ID and fill it in when you answer the original ticket raised. You can even have your own rule for an RMA ID. For example, you could use a category prefix, and the year, week number, and ticket ID e.g. ATS-25320042184 for this ticket here. You cannot automate this unless you write a custom plugin handling the onContentAfterSave event. Or you can forego custom code and just fill it in manually before replying to the client. I suppose that the whole point of them filing an RMA request is that you have to get back to them before deciding whether to accept it.