I did that with one of my alts! I used the mossy carpet and garden wall and made it into a Japanese-style tea room/bonsai garden. It's a pretty relaxing place in the house.
If you're into art, an art studio. I'm planning to do this with my main room, make some designs of partially finished paintings and have them on easels around the room. With items like the fruit basket, vases of flowers, a desk, painting set, etc.
All of my other rooms are complete as well and I haven't gotten started on this yet (too lazy to make the designs atm).
Your house theme so far seems to be the typical house, so maybe a dining room like what LambdaDelta said, or...
You could have a playroom for a little kid, or a room with instruments and stuff with a teenage garage band type of feel, or maybe a man cave (lol) with a pool table, dartboard, and bar area?
I like the garden idea. You could also make a guest room using an unorderable furniture set that may be taking up storage space and that you don't want to get rid of, if you have any like that.
I always keep my main room as a room for all the seasonal stuff each month (Halloween, Much, Jingle etc.) and add in all my usual favourite items! (Eg. My gold house model, villager pics etc.)
I just change it round each month and it always looks seasonal! Plus it keeps the room interesting, instead of walking into the same old room everyday throughout the year xD