the cards and everything else is being kept on-chain, via JSON objects, text in a specific format
hmmmm, is that really true ? what that means ? history of transfers is equal to cards being on chain ? I am not even sure if you can recreate card balances from chain, etc.
For sure custom_json is probably most under-valued thing we have here. It's great for connecting chain with external databases, allowing users to interact with it, "give orders" to scripts, lot's of possibilities for games and apps...
About staking, in your scenario, you made it separate from powering up. Do we need more buttons and options for users? If anything like that is possible it should be designed into already existing staking(power up) instead of creating separate staking for another set of rewards
Transition from pob-reward-system to other-reward-system, should be taken in steps, instead of hard core switch... Not even sure if we can remove it completely :)