More ideas:
-stay the same RNG
-Remove possibility to get purple/green stones from mobs
-Purple/green stones can be dropped from UD/TQM/events.
-Adding 2 more cards(now 5).(to be decided, you need to think about it first)
-All players are able to udgrade to +12(good defense and attack) without difficulty.
-Decreases Sucess rate from +12-15(50-80% maybe? idk) THIS WAY WON'T BE TOO MANY +15
-Zeni usage to udgrade(idk the amount, 200k ea udgrade?)
-Removing all stones from cash shop or decrease the rate a lot..
-Keep the same Red/Blue drop rate(keep the white drop rate same as well?, idk you need to think about it.)
-Purple/green usage should increase the possibity to hit sucess card(and make the stones harder to get).
-Purple/green stones gives +2-3 only, remove +1
-Removing broken card.