Forest can be dark and gloomy and autumn is very seasonal, but if considering seasons, a spring theme can be beautiful with a light blue ceiling and clouds and a sun. You can use the greens for a grassland, or a fence painted. Flowers of any types go with it. You can use a tree in bloom on the wall also.

You can even do it with ceiling light panels if there is space

http://www.artfiberglass.com/ceilings.ht...

There are many options for a spring themed room.

Although the last one you mentioned would make you room look more spacious and modern because of the white, the autumnal theme appeals to me most. It is just cosy and the warm colours are very homely. I would stick to the oak furniture though - a favourite of mine haha!

Dark green is what I currently have and I really don't like it - sort of cold and not what i imagined at first

I have always liked forest themed rooms, with oak furniture and dark green walls. But I also like autumn themed rooms, warm colours and autumn tree wall art. Then I liked one wall bright lime green and the rest white. I have quite a narrow small room with a slanted ceiling at a corner. What would look best?