I'd like to echo everyone's comments about how amazing this game is! The art is gorgeous and evocative. The writing is extremely hot (particularly the soul transformations). And the mechanics are clever and engaging, without being overwhelming or distracting from the writing.
In terms of new transformations, I'd be keen for some style/wardrobe changes (e.g. piercings, leather gear, sports gear, tattoos, etc).
I also really enjoy the existing transformations and would love to see more interactions between them. I feel like the game really comes alive when it acknowledges the existing changes (e.g. the Aroma/Insufflation interaction) so it would be great to see more of those moments. Lunar/Solar alignment in particular feels like it is rarely referenced outside soul transformations. For example Reverence seems to play out with Mark in a dom role regardless of whether he is Lunar/Solar - it would be cool if that changed when he had Lunar alignment.
Similarly I think it would be great if Fatuity/Addiction/Thirst/etc could be applied to multiple players. It feels a bit limiting to only be able to apply them to one player, especially in sandbox mode when trying to exact revenge on Caleb and Mark :)
But really, whatever you decide to do next, I'm keen to play it!