There are just two things I want to see from publishers/digital distributers:
1. The ability to disassociate a key from your account.
2. For digital "purchases," the ability to "relinquish" your right to access that title. Titles purchased through Steam or Origin (etc) can be sold back to the service for a nominal amount (say 10-20% of purchase price or current price, whichever is less).
So, #1 would be mainly for physical copies so that they can be sold on and the new user can register the key to their account. Looking at my book case I've got dozens of games sat there, some of which I haven't touched in ten years, because of cd-keys that mean I can't sell them on as anything but coasters. Probably a half dozen of those are in Steam and Origin.
Implement #1 and #2 follows naturally. Dole it out as store credit to keep people locked into your service. There are a dozen games in my Steam library that I'd give up in a second, if it were possible and if there was some kind of incentive to do so. Even 10% back as store credit would be incentive enough and I'd be far more likely to purchase more through them going forward.
Oooh, I've thought of a 3rd thing...
3. Subscription to games.
Not a flat-fee all-you-can-eat kind of subscription, more like a lease. You'd buy a subscription to a title for a set period at a percentage of retail. Say 1 month at 50%, 6 months at 70%, 1 year at 80%, and an unlimited duration at full retail price (which is basically how the service operates right now anyway). Could even have the longer term as standard and have "sales" where you can get it for 1 month at a bigger discount. Yikes, I've just turned every game ever made into an MMO
Think about it. Why pay full price for a game you'll finish in a few days and not go back to for possibly years? Think a £35 title that you can have for 6 months at £25. You have your fun and it disappears from your account when the time runs out. A few years later you fancy another go and see if in a sale at 1 month for £5. Still hasn't cost you full price.
Think PAYG for games