Luigi's final Smash, Negative Zone, would not be akin to my idea for Peach's. In the Negative Zone, enemies succumb to reduced attack power and increased launch distance, and if they touch the ground can become afflicted with negative effects like the flower, stun, taunting, and so on. The only positive effect luigi gains is an increased chance of a chritical Up-B, he in fact mvoes slower in the negative zone because he dances.
Peach's mirror world does only one thing to the enemy, and that is give them slower movement speed and increased attack delay. Peach, however, gains increased speed and and an unlimited float timer, with faster and high power turnips. Turnips pull up faster, have much larger hit boxes, and deal more % damage and much greater knockback. The chances of the Stitchy turnip, the Bomb, and the Beamsword appearing are increased.
I like this final smash because SMB2 is what made PEach what she is now. It gave her the floating, the signature blonde hair and pink dress, and it's where she gets her turnip ability from. Peach's final smash should, as such, reflect this by also being based off the same game that made her playable.