I too hope the Devs really think and consider the ramifications of just opening up new races for everyone. But I will bring up a few points that came to mind while thinking about this. First of off, there will be no global chat, so once a area/tribe is discovered it will not become public knowledge unless you want it to be. Perhaps your Guild will know and your Kingdom may know but if you don't post about it or open it up in Discord, it may remain a secret for a while.
Also I like the idea of not being able to create a character using a new tribe unless you make the trip to the area to unlock the ability. It would be strange indeed to make a new character and be transported to a area of the world I have never been to before.
Now I feel your fealty will depend on the individual player. Is he exploring for the Crown or is he just looking for a personal adventure? Will he claim the new land as part of his Kingdom or will he keep it a secret? These new areas are owned by other kingdoms and taking the land will not sit well with them. However if you keep a low profile you may be able to keep a small community secret, or you may pay a small tax to the new kingdom while staying loyal to your old one. There could be a few possibilities that could arise from this situation. Now I feel you won't be able to fight off the new kingdom once you are found out, especially being so far from home for support, so you need to find a unique solution for this situation.