Hi Paul,
I really like the smaller form factor of the T4.0 - my only request would be easier access to the SDIO pins, but I am managing fine so far. I'd probably be very interested in a 4.1 (3.6FF) variant it anything became available.
If you were doing a bigger version, which the RT1170 would have to be, which is heading into SOM territory, here are the things that I thing would make a difference to me:
-Expose the DSI and CSI - could be high density connector for a mezzanine etc. The 1170 will be the first affordable bare metal, Arduino compatible micro that has a fully functional MIPI DSI interface, there are so many great, cheap screens that are not accessible to hobbyists because there has never been a good (relatively open) controller that supports MIPI DSI. Most parts that are available don't have the required documentation, exposed interface or are many hundreds of dollars. I don't know the architecture or how the peripherals would be mapped, but the idea of the co-processor just taking care of graphics and the main core having a processing routine would be amazing. The specs have 1080P @ 30Hz which would open up a whole new range of ux and embedded display ideas.
-In the use case above - a single 1080p frame at 16bpp is ~4Mb - so something like 16Mb of memory mapped ram (QSPI?) would allow a decent frame buffer and scratch space.
-On-board SD with UHS-I compatible interface.
-I like the ideas of a mezzanine of the small main module (Edison, or any other SOM really style connector) and a breadboard compatible breakout board, if we want to integrate this part into our own products this really improves the situation.
-USB-C connector - they now don't add any cost to the BoM over a micro usb and are really so much better. Follow the rules and they are bullet proof.
-OTA firmware updates: I know this has been a tricky point in the past, but how well it works on the ESP8266/ESP32 really makes that an amazing package if that is your requirement.
Keep the Teensy bootloader closed-source, protect your IP and the products from being copied, that is 100% reasonable, and has resulted in an amazing product, community and has kept PJRC a leader in the space.
My Proposal; if it was possible;
expose a TTL serial port on the MK02 with a documented protocol (half-kay?) that allows flashing of firmware, Eeprom, fuse configuration (where required) and reboot of the main Teensy uC.
For those that are interested, they can interface with this with a hypervisor/loader of their choice. ESP8266, ESP32, Any Ethernet Enabled uC, Any CAN enabled uC, the implementation is up to the user, but there is a possibility to do it without having to use USB which is where the problems start.
Any number of end user OTA requirements can be achieved then, and I am sure that this would be utilized by many - myself included.
Overall the biggest thing for me has been support - which PJRC/Teensy excels at - the implementation and library support is fantastic, as it the community. So you could probably ignore all of the above and keep doing what you are doing and I will still buy your stuff.