Suggestion
I suggest that Mass & Energy Storage be given a command card toggle button to absorb mass/energy with a certain buildpower.
Another button will instantly release all stored mass/energy into your mass/energy bar.
Mechanics
Per structure, I would suggest around 25 buildpower for 5 mass or 10 energy per second.
For the storage, it would store the same amount as is listed for the mass storage structure.
Preferably, the 'global' mass storage component of the structure would decrease as the locked mass stored increases, if this is technically possible.
(and the same for energy storage)
Reasoning
This will allow you to easily store mass for when you need it, for instance to save up mass and energy for a quick T3 power plant. It could also be useful to prepare an engineer drop so you will have full build power when you need it, or an energy reserve to power your shields during an attack.
I will admit that this feature would be used more below 1500 than above it, but I still think this would make a very useful feature.