Hey, so I've been thinking about this for a bit as well.
What if the modifications were broken down into two tiers, and the mods in tier 2 are only unlockable if you have a certain combination of tier 1 mods active?
As an example:
Tier 1
- Breast Expansion
- Ass Expansion
- Height Increase
- Height Decrease
- Submissive
- Dominant
Tier 2
- Bimbo (Breast Expansion + Ass Expansion)
- Milf (Height Increase + Breast Expansion + Ass Expansion)
- Shortstack (Height Decrease + Breast Expansion + Ass Expansion)
- Muscle Girl (Height Increase + Dominant)
+ Special Mod: Animal Transformation (requires at least one mod beforehand)
- Dog Girl (Submissive)
- Cat Girl (Dominant)
- Cow Girl (Breast Expansion)
- Bunny Girl (Ass Expansion)
Don't think that is all realistic, as it would require a serious restructuring of the game, plus a rerecording of quite a few voice lines, but I just wanted to throw it out there. Maybeat least some of these ideas can help.