eMusic uses a subscription method for payment which involves paying a certain amount a month which gives you access to x amount of music. If you go under this amount, it doesn't roll over into the next month (which is perhaps a downside to using eMusic).
How many people would consider subscribing to GOG for (say, bimonthly access to) a number of games? I presume the incentive would be in dropping the price you effectively pay (i.e. pay as you go means 10 dollars a game, subscription means 7-8 dollars a game) for a game, with a possibility to pay as you go if you run out of 'game credits'. eMusic is very smart about its reaction to users going over their amount of songs per month, offering them instantly an upgrade to a wider (and more expensive) plan.
The parallels are there, eMusic sells DRM-free music, GOG.com sells DRM-free games. Perhaps GOG.com needs to expand its collection before launching a possible subscription plan (or it could offer trimester payment options, since the catalog for games is far smaller than the catalog for music anyway).
What do you guys think?