It's could happen as Rockstar have been releasing quite a lot of stuff on the IPhone.
Here are some questions
(i) Where would it be set
(ii) On what consoles
(iii) Birds-eye view or third-person
(iv) A brief description of the storyline
(v) Features from other GTA games you would like to see
(vi) Would you like it to include DLC. What type of DLC.
(vii) Any new gameplay features
(viii) Any other details
Feel free to suggest some questions. but here is my answers
(i) Los Santos. No Blaine County.
(ii) IPhone (4s and up), Nintendo 3DS, PS Vita
(iii) Birds-Eye View
(iv) The player is a member of The Professionals, and the story is about the becoming the top drug-distributor in Los Santos. Some of the street gangs are hard to keep in line, and in some parts of the story, they will attempt to destroy The Professionals. The Professionals have two main enemies also. The Triads and Russian Mafiya. The Angels of Death return causing turf war between them and The Lost. Also Da Nang Boyz return, this time being based off gangs like TRG or ABZ.
(v) Drug dealing returns from Chinatown Wars. Also turf wars return from San Andreas.
(vi) Yes and story-based dlc.
(vii) When you put something on your gps on the road it tells you weather you need to turn left or right. I've taken this from Sleeping Dogs. Also you manage drug deals and assasinations by using your mobile.