Transformers: The Last Knight opens in U.S. theaters on June 23, 2017, followed by the Bumblebee spin-off on June 8, 2018, and Transformers 6 on June 28, 2019.
Simple answer: The Avengers, of course.
Fan-made trailers aren’t new to the internet, especially when it relates to comic book and action movies.
It’s highly doubtful, borderline impossible, that fans would ever get to see a crossover event like this in theaters.