I think it'd be fun to throw in some Pokemon for those who like Pokephilia; that still counts as yiff, right? Also, how about a genderbending engine? Say you want a female horse, or a male bunny? Press a button and it's done. Thought that would give a little more variety to the models.
Comments
"No official characters will be digimon or pokemon, but a 3rd party character might be." (source: http://forum.yiffalicious.com/index.php?p=/discussion/comment/23/#Comment_23)
Changing genders would require new models. Just vote for "female horse athletic" and "male bunny chubby" in the next poll. I think the possibility to switch character models but keep the pose has also been mentioned somewhere as a negative but I can't recall/find where.
I guess I don't understand what you mean by "an engine". One of the ways to change gender - which is what I think you mean - is by removing the breasts and vulva and adding a penis to make a very feminine male or adding breasts and a vulva and removing the penis to create a very masculine female. That means making the characters modular which would be buggy as the Seven Hells and wouldn't look nearly as good. The other version - which is what I think they are going for - is making unique models for each gender of each race available with limited customisation through skins, expressions and inflation.