"An important thing to note is that the Mistlands biome will only generate in areas you have not yet discovered. Therefore, if you have explored a lot of your world, you might be better off starting a new one in order to actually be able to travel to the Mistlands."

This is from the patch notes for Mistlands. I would assume the same logic would apply for the next 2 biomes. You've got about 9-12 months before you gotta worry about it.. but I doubt anyone here is going to know for certain.