These are features we probably won't be able to add into DiamondQuest in June, but which we should seriously consider adding at later dates.
These are harmless creatures that appear randomly in tunnels. If the player walks under a spectral bat, it spits out a Spectral Seed and disappears. Spectral Seeds can be collected by the player, and then used as hints to reveal individual numbers in math problems. (There would be an additional limit how many Spectral Seeds you can use on any one problem).
Spectral Bats seem to blink into existence, and start out as being purple, but gradually fade through the spectrum until they turn white, after which point they vanish. This "fade" is controlled, not by time, but by player movement. The player will have to consider the fastest route to get to a spectral bat. Tool usage causes considerably more fade than walking and climbing, but may still be faster than taking the long route around.
This feature would contribute to the "platformer" feel, without detracting from the accessibility goals. It would also provide much needed help to players who need it.
You can add the treasures you find to a Museum. You move Treasures around the different rooms like a sort of "sticker book".
As you add to your Museum, your visitor count and publicity will go up.
The Curator of the Museum will sometimes advise you by giving you Missions. Accepting a Mission will skew the loot tables so you're more likely to find items from certain collections.
These would only appear in generated caverns. They cannot be broken directly, but only be removed by breaking their anchor block. This would add a level of difficulty, especially when drilling, although blasts would go "through" them.