I actually tend to get my room ideas from pictures of real rooms. I don't necessarily go for the color schemes in the pictures (in fact, I almost never do). I also try to make sure I have more than one "series" of furniture in a room, and once I max out my HHA points, I get rid of unnecessary things. For example, I take the beds out of what is going to be my living room, kitchen, bathroom, etc. I also try to pick a cohesive theme for the entire house, even if it's more obvious in some rooms than others. For my Mayor's house, it's going to be 'flowers', so there needs to be something floral in every room, no matter how small.
It may help to think of the character as a more fleshed out one. Figure out their likes and dislikes, their favorite colors, what the purpose of their home is. Is it even a home? Is it a store? An entire shopping mall with different sorts of stores? Is there a food court, maybe?
In my Mayor's case, The house is just a very large home for a single individual. She likes inviting villagers over, but only before a certain hour. Once night hits, she wants her privacy. As a result, the kitchen and living room have lots of seating for guests, but there's only one bed in the house because only one person will ever sleep there, ideally. Thinking about those kinds of things can help you decide on your rooms. People say limits shouldn't be placed on creativity, but I think that giving yourself limits actually helps you find more creative ways around something.