April 2026 marks the 25th Anniversary of Animal Crossing in Japan, with the US not technically hitting 25 years until 2027. 25 years tends to be one of those events that Nintendo usually celebrates. So, the question is, what do you expect to happen?
Personally, I want to believe we'll at the very least get a new game announcement, though a release in Fall 2026 would be believable too. If not that, then an NSO release of the first game would be expected. I'd fully expect new merchandise at the very least!
They could announce a new game, but I don't expect it earlier than 2027 honestly. As you said, the 25th anniversary is not the same worldwide, given that Animal Crossing first released in Japan only, so they may also not say anything at all and celebrate the anniversary in 2027 or in another year. However, I have found a post from Animal Crossing's Japanese official account which celebrated the series' 20th anniversary on April 14th 2021, which implies the 25th anniversary will in fact be in 2026.
In my opinion, the anniversary is a great occasion to announce a new game but I don't expect Nintendo to release a game in that year. If they do announce a new game, I will be very happy if we get to see some of it instead of a generic announcement, but I think that is not very likely.