A Neighborhood Alive: The Apartment Board with its many windows is a character in itself. Each window is a potential customer, and the art needs to give the building personality.
The Flowers Themselves: This is the star. The six flower families need to be distinct, beautiful, and stylistically consistent. Illustrators will need to design the look of the Tulips, Roses, Sunflowers, etc., for this world.
A Parade of Personalities: The Neighbor tiles and Player Powers are a dream brief for character design. Each should be a unique, memorable person from the neighborhood (e.g., "The Photographer," "Autumn Witch," "Good ol' Pat").
Charming Details: The Billboards, Garden Goals, and Bloom Schedule are all opportunities to inject more charm and world-building into the game.