- Should there be a tamed unit?
Eh, if you're going to add tamed units, I would actually like to see this consolidated into groups. For example, I don't believe there needs to be a specific tamed unit for each rhinocerous (although what I'm talking about will be broader than just variations on one animal). Instead, there is just one unit that you can build, and the ability to spread more of a given building/myth is allowed to the unit based on what's pre-existing in your civ.
For example, you'd have a tamed domestic herbivore unit. This unit could then build any of the common dosmetic herds in your city, provided you had such a herd (or resource) pre-existing elsewhere at the time of construction. So horses, donkey, sheep, pig (deer? Missing any?). And have separate, single tamed units for different categories: e.g. tamed big cat, tamed exotic herbivore (rhino, giraffe, zebra, maybe deer, maybe kangaroo if not split out, etc), tamed ursine (all the bears), tamed canine (although I think there is only one 'civilian' dog unit - the tamed wolf and the breeding pair wolf that the tamed doesn't obsolete - as opposed to the military units). Tamed small mammal (none exist at the moment, but would include things that can build marsupial and rodent cages). Tamed primates. (List here is not exhaustive.) These tamed units could build animal-specific structures provided that animal is present in your civ (a tamed herbivore unit could build a giraffe standard if you have giraffe myth), as well as more general buildings like herbivore cage.
It is similar to the concept of bards or its upgrade being able to build any story/stories provided you had the myth/story in the city you're using it in. However, I was going to suggest a difference too. In Fall From Heaven, spells were handled by adding promotions to the unit based on existing resources (possibly at any time provided it visits a city, or possibly only at construction/upgrade). This allowed the control bar to only show the spells it "knew" instead of showing them all but graying out the ones that were unavailable (as happens with bards/entertainers and the stories). If at all possible, I would suggest a similar scheme for the tamed units in order to help reduce the sheer number of control buttons a unit might have.
(I also wouldn't mind seeing that scheme used for the myths/story/stories of educator units so as to reduce the extensive list of build orders they have, that causes a slight slow down/freeze whenever you select them in a city. However, their list is so extensive that changing it might instead cause slowdown when they are built and their promotions are all populated - I don't know how the performance issues might compare. Also, it would require adding new promotions that get added to these units, and potentially the code to handle it all if it is not already existing/available in C2C.)
Certainly I can understand it is much too late to make such extensive changes.