Some players can't play every day, so it would be neat to see an item catered to players who can only play a limited number of times weekly.
Enabling players to expand their mist energy capacity in a way would help these players, and unlike the popular alternative (Excess ME goes into CE), it keeps CE to be something that requires money to be introduced into the game world.

I think that an expansion would be best implemented as an energy-costing upgrade like slot unlocks, and it should fill out over a week or so.
This expansion should:
Last 30 days like slot upgrades.
Add +200 to +300 capacity, maxing out where crafting 4* equipments is far less costly and 5* equipments are still expensive, while allowing about a good amount of dungeon delving.
Give a penalty of -66% to -50% fill rate when you have 100 ME or more, thus making the expansion more of a utility for not-daily playing than a way to actively stockpile ME. The 100 ME threshold means that you could still play daily if you feel like.
and most importantly, It should not be prohibitively expensive -- 300 to 400 energy cost. This is cheap enough for weekly players to get a lot of extra energy to use at a moderate cost (a pretty cheap cost per week!), and is not very useful for daily players. While one could use it to bypass having to use CE to craft 3+* equipments (and in fact save CE doing it this way), it would be a very slow process involving a lot of not-playing.

Questions? Comments? Glaring issues? Please reply!