At this moment I've considering 3 shields...
1: Speaker amplifier with I2S input, plus NAND serial flash (64 Mbyte) - similar to the prop shield, but meant for Teensy 4.0, and without the motion sensors.
2: Microphone and WS2812B 5V buffer
3: Lipo battery charger & power management (maybe in 2 versions, with 5V boost & Teensy runs from 3.7V)
So far, what I'm seeing in this thread sounds like breakout boards. Is that right?
Interesting shields, though I probably would prefer one shield with all 3 options.
I go back and forth as to how to provide battery charging to the Teensy. In the past I've used onehorse's. My problem is it doesn't fit into using other shields because it only occupies a portion of the Teensy volume. I.e. if I mount it on the bottom, the Teensy doesn't sit flat. If I mount it on the top, I can't use other shields. Also the JST connector with only two pins is not very secure when trying to remove batteries. I have to be real careful, not to rip the JST connector off.
So most recently, I've used the Adafruit feather adapter, which provides a passive charging system (i.e. you don't have to cut the VUSB -> VIN pad). I haven't tried out the Teensy 4.0 on/off switch to see if the Adafruit feather adapter would still charge the battery. If so, that is a plus. But from a prop usability stand point, the feather adapter has the JST connector is very when secured, and there is no problem removing batteries. While being able to mount some of the feature wings is nice, for displays at least, Adafruit misses the boat, in that on the Teensy, you have to mount the feather display underneath the Teensy.
Presumably the 5v boost version would work with the USB host pads, and bring that out.
Having the microphone and speakers on two separate boards seems to miss some common cases. Having a microphone and a level shifter allows you to do the displays based on the background sounds, but it misses the case where you might want to do a sound change. I've been noticing the Monster M4SK from Adafruit has a microphone input and speaker output, and it is now getting the initial ability to do vocal changing (i.e. 'dalek' speech). This might be better for the full audio shield, as there are comments that it degrades the performance of the M4 in the M4SK.
As I've said before, with my brief usage of some of the Adafruit boards, it is so much simpler to have the microprocessor export a removable file system, than to use teensy transfer to copy stuff to/from the flash. Of course, a lot of times this is to be able to run CircuitPython, which I have very little interest in.
In terms of the existing prop shield, if you are going to make a prop shield II for the Teensy 4.0, I would suggest bringing out the pins for the WS2812B's and speakers in a continuation of the existing 14 pins (with the speaker pins on one side, and the WS2812B pins on the other), so that it more easily fits into a standard breadboard or prototype board.
Given the T4 doesn't have touch sensors, another thing to think about is maybe bringing out some touch pads that would work via I2C. I must admit I've been fascinated by touch panels, but I've never done anything with them except a few tests.