I think a maximum mist energy tank upgrade that you can purchase (either with real money or with crowns) would be nice. I don't have many opportunities to play the game, but when I do, I like to play for long stretches at a time. The 100 mist energy per day system works well if you play every day, but it feels unnecessarily limiting if you don't have the option to. If I could have a larger energy tank that took 2 or 3 days to fill, I'd totally go for that. Instead of making it a maximum tank size upgrade, it could even be another item that you equip with a fair size/charging speed tradeoff.