Every province has basically 1 climate which is why there is one major race in each province except cyrodiil. It has the climate of black marsh/summerset isle/and skyrim which is why it is evenly distriputed among races. It is also the biggest province and is perfectly in the middle of all of the provinces. Many wars took place there which is why it is so diverse. It is the most diverse land in terms of geography, climate, and race. When races migrated they migrated in towards cyrodiil. There is almost no races in any other province except for the main one that i listed except for high rock. In morrowind we could only play as dark elves because that was the majority of the people. In oblivion we got to choose because in was evenly distributed. in Skyrim some races can't even survive because of the climate which brings me to my point. I don't think that we will be able to choose what race we want to be becuz there isnt a shred of evidence that supports that their could be other races except for dunmer because they were drivin out of morrowind at the time where skyrim takes place (i could be mistaken on that one). I think we will get to choose which type of nord we want to be like from which clan but not which race. Someone please try and prove me wrong i wanna be a dark elf they are the best. below is where i got my info.
http://elderscrolls.filefront.com/info/province2