That's a very good question. I'm not sure most games today will be revived and given new life in 20 years like the retro classics of the 80's have been revived.
However, with technology likely to be at an unimaginable advanced level in 20 years, I could see maybe taking cues from the Wii and interactivity to let us literally be in the game.
Either by interacting directly with our brains or by some advanced version of virtual reality.
This is a good question for developers to keep in mind. They should ask themselves if they truly think the game will stand the test of time and maintain popularity over a 20 year life-span. This could lead to much better games.