Thanks, I like it to.

In theory Usagi could grant herself legs? Or the transformation have to resemble the original body?
We don't see ninjas turn to animals, for example.
Well here is the thing about the mechanics of how jutsu work:

in addition to the five elemental natures, there is the yin release and yang release natures. these two natures are also universal. yin release is used in genjutsu because it controls imagination. and due to yang's physicality people like the akimichi use it to increase their size, so to me, its also the chakra used for taijutsu in general.

what do you get when you combine yin and yang? Ninjutsu, because your basically taking a genjutsu, an illusion that doesn't exist, and adding yang chakra that gives that form and physicality. thus every ninjutsu can said to be imagination made real.

thus when you use transformation jutsu- its what you imagine with yin, plus physical energy of yang. thus whatever you imagine is what you become. its classified as a ninjutsu, not a genjutsu. because ninjutsu, affect the physical world.

and again, Naruto became a giant windmill shuriken that was thrown by Sasuke in Land of Waves arc. I can look it up for you if you want, but turning into an object is pretty much anything but resembling the original body.

while Kiba used a variation of the jutsu, a transformation on his DOG to turn Akamaru into HIMSELF during combat against Naruto in the Chuunin Exams. why would Kiba do this? Naruto isn't in a situation where he can be deceived by Akamaru's disguise, the deception and confusion value is limited and pretty much worthless in a duel where your foe knows both things coming after him are enemies and thus know that taking out either of them is a good outcome, and the dog will reveal that he is a dog through his actions anyways. the only explanation is that transformation jutsu somehow enhances Akamarus combat abilities for it be worth using.

so technically yes, Usagi COULD use the technique to have legs. it would just be a drain on her chakra constantly maintaining it.

and technically yes, a person can transform into an animal. sure it never happens with transformation jutsu, but the reverse does and if Naruto can become a giant wind shuriken that is then picked up and thrown, and still be alive to like, transform back, why not turn into an animal?

furthermore, if something like shadow clone can just make an entire temporary clone of you, completely physical and able to do things just like you, a complete person that does all the things any other human do, why wouldn't transformation be able to temporarily make a different body structure physical as well? its the same principle just on a smaller scale.

and even the elemental natures themselves aren't really outside the mechanics of yin and yang in yin-yang occult mysticism, the elements are just different combinations of these two forces, with one or the other being more prevalent in some manner so its all bound by the same "yin imagination + yang physicality" logic. going by that , the only reason anyone has an affinity over one element or the other is because of their own internal balance of yin and yang aligning with it. so if ninjutsu can do all these other things, why wouldn't it be able to make transformation an actual physical thing? and yang chakra alone like, makes chojis punches bigger.

the only downside of course is that your chakra is limited and you can easily poof out of if your concentration is disrupted. and that you probably need to like, really master transformation jutsu to its fullest to like, transform into some crazy combat form that you just maintain with pure will power while fighting someone, or just plain make a jutsu derived FROM it that does that, but takes more chakra to use even though its sturdier.

and in case your wondering why no one thought to use such an incredibly awesome super-useful jutsu to become anything even physically more than they did.....don't worry, the rest of the community who analyzed it has wondered the same thing for years. The real answer like most any shonen anime is "plot holes/author didn't think of it".