Modernize and modularize the Arduino Uno platform. Make a processor daughter board platform with height below the normal shield headers. Add voltage level shifters as appropriate for the processor.
Create a better USB to serial/SWD programming and debugging capabilities using Blackmagic Probe firmware. (Off board programming support with a connector as well.)
Support common WiFi, Bluetooth, and other microcontroller modules soldered onto the daughter boards as well as many non-module microcontrollers.
Support direct power from 2-16V using a newer power supply technology.
Possibly support USB Type-C power as well.
The idea is to have one Uno like clone that supports multiple microprocessors by daughter board and most Arduino shields.