Before I get started, I want to get this out of the way now because I just know some idiot is going to skim the post and then loudly complain about something I did not say: this is not in any way, shape, or form a means to spend more than 100 mist at once; you still require CE for any such expenditures and I LIKE it that way.

Now then, this suggestion is all about the unfortunate side-effect of the daily mist limit. I understand why it is there, and I agree with it, however the current implementation of it needlessly punishes players who don't log in and use all of their mist every single day. For a casual game, making somebody feel like they need to login every day is a good way to turn potential customers away. Easy solutions to this problem, such as increasing the amount of mist you can store, aren't feasible because they break the game balance (by allowing you to buy things priced over 100 energy entirely with mist for example). However, an idea I hit upon last night when somebody mentioned this problem to me is that perhaps the amount of mist you can hold at once could be extended without breaking the balance the devs have created. I do believe this is the solution to the problem at hand.

Reserve energy. Any mist gained over 100- whether naturally or from a mist tank- goes into the reserve tank. When you spend energy, it still comes out of your regular mist energy, and if the total goes over 100 then the rest comes out of your crystal energy. After the transaction is done, mist energy is refilled from reserve energy as much as possible (up to 100 or whatever is left in the reserves). This way, a player can effectively bank multiple days worth of playtime but still requires crystal energy to do anything that requires more than 100 energy at once. This avoids punishing players for failing to login every day while still keeping in place the established daily limit on playtime and maintaining the current game balance. There is also a side benefit that it's easier to use mist tanks without wasting some portion of them. The size of the reserve tank is up to the developers to decide, though I would ask for it to be at least three days worth (300), preferably closer to a week's worth (700).

tl;dr: Extra mist over 100 goes into reserve tank (developers decide how big this tank is). Reserve energy can only be used to refill mist, not for anything else (so you still require crystal energy for anything that needs more than 100 energy at once). Players are no longer punished for not logging in every day, but game balance is preserved.

And once more just for the record: this does not allow you to spend more than 100 mist energy at once, you still require crystal energy for such purchases.