Hm, I am not sure, if Paul should start adding peripheral features to the Teensy that are completely unrelated to the CPU and its environment. You need a Lipo Charger, another one needs a little piezo speaker, someone wants a step motor driver and the next a little microphone. Why isn't there a 6N138 on every RX Pin, so that we could directly connect de-coupled midi (a feature I have on over 90% of my Teensy Projects, but I assume the majority wont need). Why do we have to add an audio shield to the teensy, when Paul could add the codec chip to the Teensy itself? Why isn't there a LCD driver directly on the board, so we could use displays without them in our projects? Wouldn't it be awesome if there were at least 4 high quality DACs on the board? I cannot remember the last time I did a Teensy-Project without the need of multiplexing IO. It would make my projects a ton more easy, if there were a few multiplexers directly on the Teensy. But wait, how should Paul know what multiplexers I need, and what if the next person would rather use a shift register instead? What do I do, if Paul adds a Battery charger for 1S with a charge rating of 100mA, but I need 12V in my project somewhere, go with 3S Battery, maybe even quite big, and 1S charging with 100mA wouldn't help at all? Or maybe still 1S but the 100mA charging that might be perfectly for your project, is way to low for mine?
You see where I am going to: I am sure nearly every single person would like to have different peripheral on the teensy. So the simplest solution is to focus on what is directly related to the CPU and its directly related components (like memory, IO like USB etc), keep the costs as low as possible for the board, and let the maker decide what additional stuff is needed and choose what specification it should have.
For me personally, I would rather love to see more IO available, bring back USB Device Connection etc. than "wasting" space on the board for features a lot of projects won't need but everybody has to pay for. And while it is somewhat awesome, that there is USB, SD etc. on board already, when it comes to designing my PCB and start integrating the Teensy into my project, I feel that having all this on the board puts a lot of limitations into the design (It is hard to make USB and SD Card available in projects that are bigger than the length of the board without extension cables). I would sometimes love to have more available juice on the 3.3V line, but as Teensy is designed to manage the power regulation and prefers to not get powered on the 3.3V pins, it is not really possible to use an external one.
If I could name a wish: I would love to have a teensy, where as much as possible IO is available as headers, because adding connections to those solder-pads is awful in a project. Like, 5.0 in small, 5.1 in big, 5.x in big, but without all the additional stuff, rather add a second row of pins
For Battery management, if you dont already know:
Teensy already comes with everything you need to add an external battery with charging to your project. On the bottom of the board, where the USB Connector is, are 2 rectangle pads with a small connection between them. By cutting it, you disconnect VBUS from VIN. That way, USB-Power doesn't connect automatically to the voltage regulator on the Teensy. Now connected the VBUS Pin to the IN-Pad of the charging/bms-board of your choice, connect the VIN Pin to the OUT-Pad of the board (make sure you have one that delivers 5V) and connect the Battery of choice to the Battery Pads of that board.
And the awesome part of that is: YOU can device what batteries you choose, how much power is used for charging, maybe even use your own circuitry on a custom PCB for that project.