There is a particular skill needed after beginner levels that could really use a mini game.
It should start out with just open strings, then add chords you have to fret while still picking the right strings,
which are random.
Slowly the chords change more often and the speed slowly gets faster.
It will develope better muscle memmory for picking the right strings quickly and
develope the concept that many single note riffs are in fact chords that are single note picked.
Just an idea
Rock On !