Minicraft+ has a lot of additional features. Adding them wouldn't be impossible, but it would take quite some time to get right.
To make this demake I had to learn how to program the GBA, and plan a lot of things beforehand (eg. the save system is made for exactly this number of tiles; adding new tiles would mean change a lot of the save system).
There is a lot of space for new entities and tools though.
As for how it was made, I had to rewrite the game from scratch.