Future Dragon Ball Super trailer analysis
In my opinion no, Broly can not be placed anywhere in the dbz manga story line, cause it would have been somewhere before the cell games. But yeah her transformation looks a lot like Broly. We should wait for more information, this could be a real "legendary" transformation or just something that resembles it. There is a lot of "fan fiction" that was taken from community response (like Veggetto) so that could be the case with the legendary transformation as well ( because it became iconic)