Spark Core has one problem at least for my perspective.
It does not show how much spark you will have after the coal has been burned out from there.
Sure i know you can somehow calculate this with heat and all in the game atm , but i think it would be more convinient that it would show the amount you end up with.
Like you put 200 med coals in and it would show “(spark after refill xxxxx)”
This is not mandatory and i would not but this like on the top of your list, but at least for players like me who have kids , works and stuff it would be nice to put coal to spark core before quitting the game knowing i can start crafting stuff when i enter into the game next time.