In order of descending levels of eroticism, I would probably…
- Have them in a foursome with their partners and do a Midsummer’s Night Dream set up wherein the transformation vector, which is desire-based, ends up fixed on the wrong partner and the girls swap for some consensual cheating.
- Have this be a non-sexual D/s scene where the transformation inverts their power dynamic.
- Revisit one of my favorite episodes of Gargoyles and have one or both of them turn into something demihuman and go on adventures.
Personally, I want to write the last one the most. In part, because I do not feel like I write enough (P)SFW TF stories, and also because I just realized that I have done exactly zero riffs on Gargoyles.