Thinking a bit more, for the second rev of the hardware, I'd like to add a goal to be able to perform JTAG or SWD debugging.
It appears from various forum entries, this isn't explicitly supported by the currently available Teensy boards (although it seems JTAG support (with a header) might be coming at some point in the not too distant future). For the time being, the primary goal of the Teensy hardware seems to be to support the Arduino IDE, which I suspect is likely the vast majority of Teensy users (I'm likely the oddball here).
I have found one guy who did some serious reverse engineering to be able to use the Teensy 3.1/3.2 with his JTAG, but it requires cutting through the PCB substrate to gain access to certain traces. I'm just not super interested in going to those lengths to support JTAG.
On the Teensy 3.6 on the other hand, it appears there are a couple of pads on the backside of the board intended to support SWD debugging.
In this image, on the back side of the Teensy_3.6, there are pads for DD (Debug Data) and (Debug Clock), and a third called DE (Debug Enable). These are SWD signals. It seems the purpose of the DE pin is to allow an external debug device to pull that signal low, and disable the chip responsible for communicating with the Arduino IDE, essentially allowing the external debug device to take over control.
Unfortunately, it seems the DE pin doesn't quite yet work as advertised. The following blog provides some additional details on adding debug support on a Teensy 3.6. Summary: It can still be done, but a warranty-breaking removal of the Ardunino communication chip with a heat gun, which I don't yet own, will be required. I don't necessarily mind "breaking the warranty", but I'll need to find a heat gun.
Attaching Debugger to Teensy 3.6
In other words, the next gen "sensor board", or whatever it will be called, will be built using a Teensy 3.6 and not the 3.2.
The probe I have been looking at is this one. Its relatively inexpensive.