That development would be another "hundo" Mewtwo, dubbed as an "anniversary special", which all Trainers could get over the past month (through a Timed Research set) with all the same conditions of the one from the Times Square event but with a different background; those who partook in said event get one with random IVs instead. Its potential to quell certain controversies is also offset by its potential to continue generating them.
For instance, some Trainers became angry that they got it after they had used a Bottle Cap to max out a recently or previously caught Mewtwo. A certain Trainer has even shown themselves transferring said Mewtwo with the pretext of defending the ideal of needing to work for it. Even so, this may be less of a controversy and more of an evidence that these Trainers are in fact an ungrateful lot, something that should be discussed separately.
Meanwhile, some have also postulated that as a result of both "hundo" offerings, the value of Mewtwo has been reduced, and what may be expected "for all time" is a way to obtain such a Mewtwo, especially for Trainers who missed either offering above due to their quite inherent limitations. Of course, they all will still need to raid for resources to power up the Pokémon, so the reduction in value may be offset by the continuous resource need.
With that, continued offering seems to be the way things will go. The developers have said that the Times Square Mewtwo might just return in the future, and the "anniversary special" thus might not be far behind. Whether or not they'll come as "hundo" for those who don't already have them (and with random IVs for those who do) as in their original cases above is not certain, but if the will is the continued offering, there may be ways.
"Legendary" is the moniker that applies for some Pokémon species because of their in-game properties. Yet it seems that for Mewtwo, that moniker might be considered applicable in another way - particularly with regard to Pokémon Go - for its awe-inspiring and controversy-generating capabilities. The latter shouldn't be "for all time", but the former definitely should be, as is the application of the moniker that is rightfully applicable to it.
One year ago: The Pains and Gains of the Local TCG Registration System
Two years ago: Cosplay: Cosplay Creator and Community Convention (C4)
Three years ago: Pokémon Go Tour for Other Regions?
Four years ago: The New (Old) Legacy Trainer Challenge
Five years ago: Pikachu Fidget Spinner
Six years ago: Item Runs in Pokémon Shuffle
Seven years ago: Remember, Remember
Eight years ago: Community Raid Days?
Nine years ago: Pokémon and Party Games