As Marco already pointed out I would love to see this feature in Aeternity. Being in the frontend of making dapps more user-friendly, one of the major roadblocks I see is the need of end-users having to own AE tokens. Which might make sense from the blockchain side of thing, but not at all from the application/end-user point of view (at least not in the current age).
If I want to build a mainstream app using Aeternity I will have to force my users to get AE tokens, which is a major on-boarding hurdle. Remember those users just want to use my app, they might not know what blockchain is, what a token is, what a centralized exchange is. Let alone that they would be willing to pass a utility bill to get validated on an exchange to finally get tokens, learn how to transfer them, to finally end up back to the initial app and start the onboarding process of the app.
I hope you understand that building an app in the current state will only attract people from within the blockchain community. Or apps will be created which will have minimum blockchain integration.
But being able to pay for a user’s transactions, allows me to create dapps without expecting my end-user to fully understand blockchain and own AE tokens. I would be able to build a game running on AE where the game pays for the tx costs and the end-user pays with a credit card to the game studio. Transaction costs would then be seen as an operating cost.
Once the players are more involved in the game you can start rewarding them back with AE tokens to pull them into the blockchain ecosystem.
At the moment we are building an extension to Arkane, and this feature is a must-have. Even though Arkane supports multiple blockchains there is currently only one that is offering this functionality.