The game is simple in its construction and does not force someone to really think about what they're doing. The only thing to beat is the clock, really - and there should be some kind of rewarding end-scene for doing so, or an extension of the star-merits, so that a seasoned player can try to beat their old score. Perhaps there should be more incentive to compete against or befriend other artists, other than merely being able to try new techniques. Even a "New Game +" option or some medals would add to this game.
As an idea, you could present something of a side-game by allowing the player to identify some famous works from the artists involved, allowing the player to gain inspiration or insight into technique by having some sense of what they're looking at. A short bio of the other artists, or even a generated bio of the player would add a little interest to art fans.
That having been said (I know... I type too much), this is one of the finest games I've played in terms of historical interest, interest in art, and even background music. I'd really like to help plan out a v2.0 . This delight will always be one of my favorites.