Regarding switches and lipo, as long as it it doesn't change the size or increase the cost! The challenge Paul has I'm sure is he can't satisfy everyone. In general, I think people would rather have to add what they need instead of deal with what they don't. The minimalism of the Teensy design is what set's it apart from most other boards IMHO.
Oh sure, I realize the challenges Paul must have faced and every thing is a trade-off. I tend to use Teensy in props, and it is better to have an on-board switch to turn the unit on/off, then string wires which typically will break at just the wrong time, etc. But it isn't the end of the world if there isn't a switch.
When I'm using something with neopixels or sounds, it is useful to have a quick on/off button. I can (and sometimes do) put the on/off button on the battery, but this means I need to modify every lipo battery to have an on/off switch.
If VUSB had been brought out to the main row of pins (moving either analog ground or A9 to the inner row) adjacent to VIN, you could simply solder a on-off switch between the two. But with VUSB being in the inner row, and not adjacent to VIN makes it slightly harder.
For a lot of setups that I do, the best would be to have VUSB, VIN, regular ground, and 3.3v all together. Then you could use regular female headers to make a quick connection off board to either supply regulated power (3.3v + ground), normal power (VIN + ground) via 2 pin connection. But this would have forced A9, A8, and AGND to the center pins.
In terms of the feather design, there is an enable pin (pin 5 on the right). Unfortunately, in the Teensy feather adapter, they don't use it. Even if the enable pin on the Teensy/feather adapter had only worked for the battery and not USB, it would have been useful.