You ask them, "Have you considered doing another story with [character name]?"

[Kadmos1]

Here's an interesting thread. In the 80+ years that DC has existed under various names, some of their characters have become public domain. Heck, some of them may have not had any official appearances (guidebooks or licensed merchandise included). The exception would be graphic novels reprints of their appearances. Those characters possibly are on the Public Domain Wiki's "DC Characters" section: pdsh.wikia.com/wiki/CategoryC_Characters?page=2. While many characters were already PD at the time DC made the comic versions, it's kind of interesting how you can now have a PD comic book version of a PD character.

Were you involved or know someone who was involved in the following situation? You are at a comic convention and you have the chance to meet with a D.C. staff member or artist/writer that is contracted with them. You ask them, "Have you considered doing another story with [character name]?" They reply something to the effect, "Do we have [or even "still have"] that character in our catalog?" Whether or not they pitched that idea to their supervisors at their next meeting is anyone's guess. With so many characters made over those 80+ years, it is possible some have been almost completely forgotten by the D.C. staff or contractees, outside of those that are hard-core archivists/historians.

It is possible some obscure DC comics (this thread can also apply to Marvel) character revived and the person who brought it up in the above situation might be the "seed" of getting them returned.