I'd like to see a version available with the Ethernet magjack and host USB is already mounted. Maybe move most of the I/O to a surface mount socket that a cable could be connected to for a daughter board since the allowed ampacity is very low per pin from the processor. Possibly a lipo battery connector with charge circuits. Exchange the type micro B socket for a type C socket on the USB device connector. I feel there is a large hobbiest and higher education student market for people that have a hard time with soldering. After all, there are very limited solutions for wired ethernet IOT right now.
On the software side, I'd like to see more work done to have fully working Ethernet libraries with example code included with TeensyDuino. Possibly induding examples incorporating things like the REST API, MODBUS TCP, and MQTT protocols. (Homekit, Google Home, and Amazon Alexa kind of goes with this, but needs a lot of extra code to handle mandatory encryption.)