I've learned my lesson after writing for several transformation-heavy games now. As with all things, the point to consider is that "if we can't do it well, we shouldn't do it." To that end:
- The PC will always have a sex of some sort. No genderless. If an item proc would leave you with no sex, either that TF will not proc or the character will refuse to use the item at all.
- The PC will always have some clothing on, and be equipped with a weapon.
- The PC will always be bipedal, with their primary genitalia in the forward crotchal region. This means no centaurs/nagas.
These can all be done away with in end-game scenarios (eg. Bad Ends), but otherwise the above are hard and fast rules. We've proven that doing otherwise results in a sub-optimal experience for both writers and players, as much as some may try to argue otherwise.
The presence of tail genitals, fuckable breasts, and multiple cocks should be considered. Support is appreciated, but not necessary. There will not be multiple vaginas, nor wombs to go along with them.