The current "pay once, play forever" scheme will be around for quite a while, but game developers are looking more and more into concepts that "lure" people into paying, for example, a monthly fee for playing a game. Online multi-player games are just the beginning. Purchasing extra content to games will be the next step. The Sims games series is a prime example of this. However rather than buying a new disc for every added part, people will be able to purchase the content online and "plug" the extra content into their pre-existing game. Eventually you might be able to download, for example, a first level of a platform game for free, and then purchase additional levels as you go on.
Source:
SPOnG