To start things off I propose that we condense our world into three islands (bodies above water, not a typical desert or tropical island). This way we have seperate locations for biomes, cultures and so forth.
Secondly, as a race of sentient fungi was suggested as a possible addition to the world, most likely found in swamps or underground, I will be the first to throw some ideas out there.
They are all linked together somehow, through connection under ground or chemical communication through the air. They most likely infest the bodies of creatures they can use, with motor functions and brain capacity enough to transport them from A to B. The fungi race could also be split into sub groups, perhaps some fungi can actually move and interact with other sentient life forms, while another has the sole purpose of consuming food to produce energy.
3 things. First one being wouldn't the term of "continent" work better for the islands? No disrespect.
Second, I'm down for this mushroom race. Sounds friggin amazing.
Third, how about a nation of skeletons? Not necessarily "hostile" skeletons, but rather treated like normal people.