The Shapeshifting Encounter

Start off the date with a casual night in, complete with dinner and a movie. As the movie plays, one partner starts to feel a strange sensation. She feels herself slowly transforming, her body starting to take on the form and texture of a pair of panties. This is her magical ability, she explains, before completing the transformation before her partner's eyes.
Following the transformation, the other partner embraces the change, treating the 'new' pair of panties with tender care. She can wear them, talk to them, and shower them with love.
Toward the end of the night, the transforming partner will turn back into her human form, allowing for a night of cuddling, shared laughter about the evening and reflection on the unique experience.

Though this scenario does not require any external location or specific costumes, it does require a touch of imagination and a dash of creativity. Make sure your home is comfortable and cozy. Extra pillows and blankets can create a warm atmosphere for this intimate date night.
The transforming partner can wear a special pair of panties that she feels best represents her during the transformation scenario. She can choose something that suits her personality or something special that she knows her partner likes.
Plan for a casual dinner and movie night. Choose a movie that both of you enjoy to set the mood and help create a relaxed environment.

Let loose and express yourself - part of the excitement of roleplay is the opportunity it provides to tap into parts of your personality you may not normally explore. Even if it seems outlandish, letting your imagination run wild can provide more fun for both you and your partner.
If the transformation roleplay is new to either or both of you, it might be a good idea to have a conversation about it beforehand to ensure you're both comfortable with the scenario. Discuss how it will unfold, what both of you expect from the roleplay, and what the boundaries are.
Since this roleplay involves one partner 'wearing' the other, the transforming partner might want to invest in a pair of panties that are comfortable and appealing to her partner. The wearer should show appreciation and express feelings as if she is really with her partner.