That came to my mind whenever Fairy type was confirmed. Hidden power makes certain type moves available to Pokemon that they would not be able to learn otherwise. With that said, I believe it has to be included--otherwise the purpose of HP will be thrown off. I see 3 possibilities:
1) IF it is included HP will be recalculated. I can see this happening since they said fairy type would add to the strategy of the game. This recalculation might change the important HPs as we know them. However, if the recalculation results in a change of all HPs, it is kinda unfair for people who already have Pokemon with a certain HP (this will affect people who play competitively in global/regional/national tournaments or just through WiFi-- a good chunk of the community).
2) Another possibility I see is that they leave certain hidden powers as they are and simply replace SOME of the lesser used ones with fairy. Like say, HP Dark (which is 31, 31, 31, 31, 31, 31), while leaving Fire/Ice/Grass alone. However, the only way this could happen is to find a certain calculation that will allow for those important high HPs to stay the same.
3) The last possibility I see is that they DO NOT change HP. What will happen to fairy? Perhaps they will make some unique move or something that every other Pokemon can learn (similar to Toxic).
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