If I'm not mistaken, Overwolf is your only shot at getting an overlay on TS3. We know it's awful. There's good reason why the community has taken a strong liking to Mumble.
You're not going to be able to find many plug-ins for TS3 because, unlike Mumble, TS3 is proprietary, meaning that the executives of the company that owns TS3 has to fully approve any plug-ins for it before they can be released and used legally per the license agreement. If you haven't found a plug-in by now endorsed on the TeamSpeak website, it's unlikely that you'll find one at all.
Mumble, however, is Open Source - meaning that its source code is open to the public for editing, testing, and reviewing. The pros of this are that Mumble has been slowly shaped and honed by a community of gamers who truly care instead of a company of coders who may or may not game. In addition, since the source code is open, it's extremely easy (and legal) to make plug-ins or add-ons to Mumble that aren't necessarily endorsed by the people who originally created and manage the programme. I'm sure if you dig through SourceForge you can find a few nifty things to decorate it with.
In fact, with a quick search on SourceForge, I found a little diddy named HAL (found here [x]) which makes Mumble look like TeamSpeak. So if it's the UI that you like about TS3, here you can have the best of both worlds - a Mumble overlay and a TS interface.
Unfortunately, your options are limited if you insist on sticking with proprietary software, yet they are sometimes more reliable and robust. Open source gives you many options, but some may be buggy or even just abandoned projects still in Alpha or Beta. In terms of VoIP software, Mumble has a clear advantage in that it's at least as robust as Mumble, has more add-on options to make it to your liking (not to mention the stock options that you want/need), and has a large community standing where you're wanting to fit in.
Your hands are kind of tied here, unfortunately. Sorry we can't be of more help, but this is the state of things.
EDIT: Oh, okay - over the course of writing all this out, I was proven to be 100% and completely incorrect by the above link. Hope you didn't waste much time reading this.