They likely didn't show her cause they didn't want to reveal her yet. I learned today that the trailer initially debuted in japan before being included on mega collection, so by the time mega collection came out and included art of cream, the trailer was outdated. Besides, if you watch the trailer, her orange slot on the character wheel is there. She was definitely already in the game, but as I said, they probably didn't feel like showcasing her yet for a multitude of reasons. You did remind me that advance 2 did have that music in the trailer... Odd thing is, everyone calls it the "Leaf Forest Beta" music, but do we actually have confirmation that it was meant for leaf forest?