Do you think there is any way Steam could have tradable games? Perhaps there could be some limits in place that might incentivize people to buy "new" copies. Like achievements and trading cards being disabled, or even not being able to go online in the game. DLC not being saleable. Just throwing out ideas here.

If the future is to have no physical games, then the secondhand market won't exist anymore. It would be cool to get some sort of semblance of that in digital form. Publishers and Steam could even get a little kickback on the secondhand sales which doesn't have with physical media.