The "one drawing a day" is a good idea and it can be usefull to have a large collection of those.

some suggestions:

-adding some mild shadow gradients or mild color gradients like you did for the cartoon king nose it'll add some volume to your characters.

- maybe focussing, in a first time, on the classic classes for a rpg (preist, warrior, ...)

- don't forget the bad guys too. In most games we have to deal with them !!

Good luck for your challenge !