Debatable, if the new race/races had an entirely different entrance into Skyrim as oppose to the current one it could work I guess, but in many ways the game, from a racial point of view is fine, I personally aren't starting the game again, been playing the same character since release and believe it or not have only explored just over half the map, I haven't fast travelled once and never will.
And you would mind if you saw some sort of invasion/immigration of races into Skyrim?
Even if you don't start a new character, I think a big immigration might spice up the game a lot and make the world feel very rich.
Also, yeah it might make sense for an alternate ending. Why not, it's DLC right? Maybe... you come on a boat to Skyrim full of immigrants... and then Stormcloaks ambush you all when you come out... then some of the other immigrants die but you make it out... and someone tells you to flee to Whiterun! Not sure how to include Alduin, as this would probably take place in one of the Northern cities....