Well, looking at my game library and the current gaming market, I see that many of them are just "showcase games" or "playable movies," disposable games made to be played for a while and then abandoned, and many people end up having several of them unnecessarily in their library, which is my case.
I currently have 97 purchased games and barely have time to play them, so an interesting option for Steam to implement would be a system for reselling access keys.

Game publishers and developers could voluntarily participate in this system and list the games they allow to be resold. This system would be useful to generate greater turnover of games and also extend the life of many of them, after all, there are games that were released a long time ago and still have a high value today, which consequently reduces the number of sales, since many will not want to pay a high price for something old without a good reason. A resale system could revive these games and also reach a new audience.

In this system, basically, each user could resell any game from their library that is applicable, for a maximum value of up to 80% of the game's original price.

To buy, the interested party would access the reseller's profile, and below the "Games" option in the right-hand sidebar, there would be the option "<player's name>'s Shop", then they would simply click to see the available games, then click the purchase button and make the payment, then the game key would be transferred to another account and that's it.

As mentioned before, the price of each game would be defined by the user who owns the key, and the received balance would be divided into 3 parts: a percentage for Steam (the intermediary), another for the developer, and the rest for the reselling user.

Thus, games that are not selling much due to high cost or other reasons could reach new audiences, after all, many games we only get to know after seeing a friend playing them, and they would also get a constant income, even if they don't sell much.

One of the problems that could arise would be access to these keys, which would be linked to external platforms, as is the case with Rockstar Games titles, which link the keys to the Social Club. Two possible solutions for this would be for the participating company to create a system to revoke and transfer keys resold on Steam, or for Steam not to allow games that use external accounts, thus only allowing games that use the Steam account to authenticate the keys.

Anyway, it's just an idea, but it's something I would like to have as an option, precisely to get rid of several games I own, and with the balance, I could buy games that I currently want.