>>4751506I'd think perhaps something alien, which gives the most leeway/room for creativity in the design.
And maybe you could have it so that your behavior towards her influences the transformation, what sort of creature she turns into and whether she keeps her mind/sanity through the transformation.
Ideally, it should be possible to perhaps end up with her as a fangly big-titted arthropodal beast who is perfectly sane and devoted to you, or a Twilek-y Green Skinned Space Babe who wants to do it Preying Mantis-style (Complete with decapitation and head-eating).
Lotsa outcomes is what I'm reccomending, both for those who want more emphasis on the monster than the girl and vice-versa, and for those who want conventionally-attractive creatures who are a Bad End or weird. monstrosities with a Good End. Cater to as many types as you think you can.
So, any chance of you gettin' it made? I'm now really intrigued by this idea, and there's a lot of good free Dating Sim engines out there.