Many years ago, I had discussed the possibility of more friend levels in Pokémon Go beyond Best Friend, along with how I would have proposed the system. That proposed system, though, admittedly could be considered somewhat complicated, and an actual implemented system might just be simpler. Now, Pokémon Go has provided the answer to that conundrum, and it in fact has been implemented for all Trainers to take advantage of after having been tested by others just a few weeks earlier.
The next friendship level above Best Friend is called Forever Friend. Trainers attain this after 180 days of interaction in total (90 more days after attaining Best Friend), which would have correspondence to my proposed Ultimate Good Friend level, but without the Lucky Friend requirement. Its XP bonus is the same as that of Best Friend (150 thousand), and interestingly, it can be attained again in connection with another feature related to this addition, which is to be detailed separately for all its significance.
Presently, however, no one has attained this level. That's because everyone starts with the friend level that they have attained to now, and even Best Friends will still need those 90 days to interact. Moreover, the testing period was also short, and the global rollout was only a few days ago. At least in this way, no advantage is conferred to anyone who is already Best Friends with others and (would) have interacted as needed; every Best Friend is essentially "starting fresh" from that point, on up to become Forever Friend.
Because of the "forever" aspect, it may also be considered that the level would be the "final" friendship level - or "ultimate", referring back to the posed terminology as above. With the possibility of attaining the XP bonus repeatedly (again, alongside that specific feature that I'll discuss when the time is appropriate for that), that also supports the consideration. With the "forever" aspect, many friends will certainly remain on one's Friend List; accordingly, the limit of friends has also been increased to 650 to accommodate this.
Friendship is always a big thing in Pokémon Go, and the addition of the Forever Friends level seems to clarify that even further. It will take some work - in the three months of this season and beyond, for both Best Friends and non-Best Friends - to get to that level, but the payoff can be considered big as well, with that feature that I keep hinting about. Friendship is also a simple thing as long as one keeps interacting, and Trainers in Pokémon Go will certainly keep doing that for as long as possible, and now when "forever" is a concern.
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