I'm not the first to ask for or dream up a Steam PC game rental or rent-to-own service, but until they offer it, I don't see the harm in speculating. There is undoubtedly a huge market of untapped potential here. Other companies have offered rental services, but none come even close to the reach Valve has with Steam. Here is my idea for a Steam Rent-to-Own service:

Rent-to-Own Model

All game rentals cost a flat rate of $1 per day, and there is no limit to the number of concurrent rentals. The $1 is charged at the beginning of the day, adjusted for each time zone, and is good for that day. If the rental is started after noon, the $1 charge starts at the beginning of the next day.

If a user cancels a rental, the game is playable through that day until midnight. If the game is currently being played, a message will pop up (non-dismissible) that tells the user their rental has expired, but they will not be kicked from the game.

Depending on the total file size, some games will have minimum rental periods, to offset the bandwidth cost. For example, a 6 terabyte game may have a 3 or 4 day minimum rental period. Game updates do not add to the minimum.

If a user chooses to buy their rental, they are charged the current Steam price minus what they've paid for the rental thus far. If at any point the total amount of money spent on the game rental meets or exceeds the current Steam price of the game (including sale prices), the rental stops and the game is permanently added to the user's Steam library.

The cost-to-buy price of every rental is listed next to the play button in the library, which is the Steam price minus what they've paid so far. Games on sale will receive special notifications to incentivize users to purchase their rental.

If a user cancels a rental, the amount paid into it is not discarded. If at any point in the future they want to re-rent the game, or buy it, the amount they previously spent will be factored in. If a game's price drops to below what a user has paid in previous rentals, and they are not currently renting the game, they can purchase the game for $1.

Users are billed weekly or monthly and can use gift certificates, in-game achievements, and other things to gain free rental time or other perks. Overdue accounts will be unable to access their rentals (rental time will not progress) or start new rentals, but their full games will not be affected.

There will be a rentals section of the Steam library, which keeps track of current rentals, past rentals, and how much money has been spent into each. It will also allow users to purchase rentals right from their library, without having to go into the Steam store.

Advantages

Rentals will reduce piracy, since users no longer need to pay the full price before knowing if they really want the game. The convenience and reliability of Steam will outweigh the small amount of money saved by torrenting. IE: If a user is only spending $1 or $5 up front, they are more likely to buy from Steam instead of torrenting, as compared to having to pay $50-60 up front.

Users will try out a much larger range of games than they would otherwise, and may allow rentals to turn into full game purchases either through forgetfulness or indifference.

Scenarios

Here are some likely scenarios under this Steam rent-to-own model:

Scenario 1: A user rents a new release, which is priced at $59.99. It has a 4 day minimum rental period. After 6 days, the user decides the game isn't worth it and cancels their rental. They spent a total of $6 on the game.

Scenario 2: A user rents a game that is at $19.99 on Steam. They play it often, but do not choose to buy it. They keep their rental going. At the beginning of the 20th day, since the amount spent meets or exceeds the game price, the game is permanently added to their library, and they are no longer charged for the rental.

Scenario 3: A user rents a game that is at $24.99 on Steam. After 10 days, the game goes on a 50% off sale (the price is now at $12.49). The user sees the cost-to-buy price next to the game when they go to play it next, which is now $2.49 ($12.49 - $10), and they decide to buy it.

Scenario 4: A new release comes out at $59.99. The user rents it for 14 days and then cancels the rental (they spent a total of $14). After 7 months, the game's price has dropped to $39.99 and then it goes on a Steam 50% off sale (the price is now at $19.99). The game is listed in the user's past rentals library with a $5.99 cost-to-buy next to it ($19.99 - $14), and the user decides to buy it.

Scenario 5: A user rents a game that is at $39.99 on Steam. After 7 days, they decide they want to buy it. They are charged $32.99 ($39.99 - $7).