A: ANYTHING YOU WANT, except swear words.
Or nothing. Some 'trailers' show cut scenes, ending with a title of a ficticious movie or TV series.
Other AMVs that end up in the 'Trailer' category are alternate openers - take an existing anime opening song, or a TV show, past or present, and then illustrate it with anime. These types of projects work by letting the lyrics of the opener do the talking.
Trailers may include overlay texts such as 'Starring [character name]' and sometimes include '...as [other character name]' if it's a crossover project.