A trademark filing for the name "City Stories" made by Take-Two indicates that Rockstar might be working on a new handheld Grand Theft Auto title.
The "City Stories" suffix is usually added to the title of mobile entries in the series (GTA: Vice City Stories and GTA: Liberty City Stories), although there's also the possibility that it could be some kind of DLC or expansion for GTAV.
It's also well documented that Rockstar are working on something for new-gen consoles, although with Grand Theft Auto IV, the subsequent story expansions came under the moniker of "Epsiodes from Liberty City".
The other alternative is that this move is simply housekeeping, with the publisher merely securing the rights to the name, just in case and/or for future use. Whatever it is, it looks like we'll have to wait a while before we find out more.