If I am not mistaken, the current way for merging stories is to wait for a mod to perform the operation. In case a user spots it, probably has to (somehow) summon a mod. This can result in no action being taken if the mods don’t spot it, or the users downvoting a relevant side of the story as “already posted” when the new link can actually be interesting anyway.
For that reason, the obvious suggestion I imagine is to add a new button to the suggest dialog that notifies the mods about the request. I know this is not as trivial to implement as a new tag, but it can be interesting under certain circumstances (like for example the GitHub and Microsoft soap opera).