First of all I did want to say that I'm extremely new to 3D editing and making TS4 content in general.
That being said though I did have an idea I'd love to be able to implement if possible. I would love to be able to make a mod that added a category to CAS - furries. I'd want to put this under the "occult" section in CAS, since they're not the same as normal human Sims. It would use the same models as the cats and dogs from the Cats and Dogs pack, except they'd walk on two legs, be Sim-sized (and possibly edited to fit the shape of a Sim, so the clothes would look better on them) and be animated the same way as an ordinary Sim. (As an example, despite the fact that they would have paws instead of normal hands, they could still use the computers, cook, etc. just as well as if they had regular hands.) And of course, be able to interact the way normal Sims do. I'd also like for them to both be able to talk in Simlish and bark/meow. If possible I'd want to keep the fur/coat editing option from the pets editor, but also be able to dress them up. Basically like a cross between the Sim CAS area and the pets CAS area ^^
Only, I have no idea how I'd do this. Can anyone give me some advice for how I would make and implement this- if I even could? Or better yet, if someone else already made such a mod like this. Thanks! ^^

Unfortunately, what you want isn't possible as we can't add anything to the CAS UI as majority of the UI is untouchable. I don't know if new voices or override (not talking about occult) voices are possible as no one has attempted it yet, but even if it was it would probably have to be applied as a trait since we can't add them through the UI. Another way you could possibly get around it is by adding in a buff, maybe, that plays a sound after testing for chat interactions (if that's even possible, I'm actually not sure if we can force sounds to play through buffs). Also, someone already made a furry mod with meshed pieces, etc. and, last I saw, people are even adding on to it if you look around. The download is here: http://www.savestatecomic.com/dlc/ Just Google furry mod for any further info.

Thank you so much for the info ^^
Kinda sucks that we can't do that but I do wonder about simply just my original using the same cat/dog model from the base game. I'll probably just start using that particular mod though because it involves less technical stuff XDD
Also, one thing I'd be interested in looking for is some way to make a dragon furry Sim, since my fursona is probably most similar to a dragon or a sergal. I'll definitely keep an eye out for more stuff that'll work with this! Thanks again

Using the cat and dog meshes would be a very challenging project - you'd have to modify them to have a roughly human shape and size and reassign the rig and bone weights in order for animations to work. It would make more sense to use a preset for body shape and/or an accessory replacement head, and custom skins.