I have successfully converted a few Teensy 3.2 and LC to work on 1.8v.
The VOUT33 and VDD rails need to be separated then you can feed 1.8v to the "3.3v". Instant power savings for those of use building data loggers.

So my suggest for future versions. Maybe LC 1.1 and Teensy 3.3.
Separate the VOUT33 and VDD with a cutable jumper and place a 1uF cap on VOUT33 for standalone regulation.

IF there is room on these already small boards!

Looking forward to seeing Teensy 4!