RE: I just had an idea for steemit. A simple feature that would allow nearly limitless functionality.
There is a test net at https://testnet.steem.vc/ you create a new account for it and then are good to go. Instructions are on the site.
The downside with storing something in the account would be that you need to get the user to give you the permission for account editing. Feels riskier than only asking for the post key. But, with steemconnect it should feel safe enough (you trust d.tube, busy.org etc that use steemconnect).
So thinking that a system would probably be something like:
User logins in in you game. You start making a custom JSON. Fill it with stuff you want to track and save. This JSON can at any time be saved to the blockchain. If something new has to be saved you fetch the last saved JSON from the blockchain, update/add and save a new back to blockchain. This way you only have one JSON that you need to keep track of. This JSON can also contain purchase info, you keep track of the block id and transaction id of purchases (game bought, in game transactions).
In the user profile you only store the block id and the json id. This should be equivalent to how busy.org saves social networks info in your profile and therefor an acceptable usage I think.
Every time a user logs in you fetch the minimal profile info and using it you can fetch the complete info for the game.
Think this should work and be without breaking any intention of how to use the steem blockchain. Think it should work to make a client only or close to client only solution to save game/transaction info or even run a Steam like service without having to worry about your own server park!
With https://spelmakare.se/steem/rps I did a minimal test doing something like this. I store the needed game info in the custom_json for the post/comments. I did have to make a server side to "encrypt" the player choices. Maybe this can be done on the client somehow to make it 100% server free. The "server" is only a lambda function on aws, making it close to 0 maintenance. In comparison operating your own server for a game, or relying on a third party like facebook, it does appear to be a promising open approach!