1. A mature narrative with deep characterization
Grand Theft Auto IV's tale of an immigrant moving to the US in pursuit of that elusive "American Dream", only to find his supposedly filthy rich cousin living in near squalor, struck a chord with a world in the midst of a global recession, paving the way for a wave of games that featured darker, more politically-minded narratives. Unfortunately, no one managed to tackle these mature themes as compellingly as Rockstar had, and so the word "gritty" became synonymous with "has a dull colour palette" on gaming websites for a few years
Rockstar has stated that Grand Theft Auto V is going to be exploring the theme of masculinity which, considering the amount of games that place us in the shoes of a handsome, manly, ass-kicking protagonist, will perhaps be subversive enough that it inspires a change in the industry wherein gamers aren't content with the heroes of their triple-A titles simply being walking pectoral muscles with grimaces smeared across their bearded faces.
Considering the huge demand for games featuring greater characterization, a void which is largely being filled by indie developers in the likes of Gone Home et al, Grand Theft Auto V might prove to be the game that gives the big companies the kick up the arse they need to get them to start taking greater care with their characters.
2. A successful balance of plot and gameplay
Very few games exist that have attempted to offer both an engrossing plot and gameplay. Obviously, it would be no small feat for a developer to be just as skilled at weaving an intricate narrative as they are making an enjoyable game, but far too often do I find myself playing through a game that's too busy with trying to deliver a "cinematic experience" instead of just allowing me to enjoy myself - even Grand Theft Auto IV toned down the fun factor considerably from San Andreas.
However, from what we've seen of Grand Theft Auto V so far it will be giving us plenty of exciting things to do in its huge world, along with what I should imagine will be an accomplished story typical of the series. If that is to be the case, and I would bet my house on it that it will be, then it will hopefully show other developers that gameplay needn't be sacrificed for story and vice versa.
3. A new console online behemoth
Online console gaming is dominated almost exclusively by the likes of Call of Duty and FIFA, but considering that everyone and his/her mother is buying Grand Theft Auto V, it's likely that Grand Theft Auto Online will soon make it one of the most-played games on Xbox Live and PSN.
With Rockstar promising so many things to see and do in the online component of the game, I hope that it will become a regular fixture of everyone's online gaming sessions, and will remain well-populated even as Call of Duty: Ghosts, Battlefield 4 etc rear their head. If Grand Theft Auto Online lives up to what is expected of it, we should anticipate that mundane cycle of FPS's and sports games to be added to in a big way.