We've touched on this before. Basically: the writing will always outpace the art, so there's no way we can ever illustrate absolutely everything without using some more generic art... which we do plan to expand. The only way for us to have a higher ratio of stuff that has art is... if I just stop adding new scenes for a long while. Not only is that mostly a waste of my time, as writing when I have stuff I want to write is the most efficient way to progress, but obviously, as we've seen in this thread, that's not a realistic option.
The ratio of new art for existing characters to new characters has been incredibly skewed of late - the last character to be added was the Dark Knight, almost a full year ago. While we are going to start adding new characters in, the amount of art and animations for each character will continue to rise, especially now that we've almost finished with the idle animations for all characters.
tl;dr there'll be more art for existing characters, and generic art and animations will have to fill in the gaps for a lot of scenes. We never planned to illustrate absolutely all of the writing in the game individually.
