The general idea here is to keep things simple and easy to use...

ATmega1284P for programming, more memory and IO than the 328P

USB-C for power and communication,

FT203XS for USB to serial communication conversion,

  -  5v power upto 1a for powering the board and sensors,

Clear markings to assist with circuit setups and programming,

Four 10-place headers:

  - A for analog ports (A0..A7)

  - B for SPI stuff

  - C for timer/pwm/serial comms

  - D for jtag and I2C

  - 5v and Ground provided at each end of the headers

ICSP port just like an Arduino

Convenient Reset button

LEDs for power (blue), serial comms (green), B0 GPIO (yellow), and error (red)

Four mounting holes (M3/#4) at the corners on a 2" x 3" (50.8mm x 76.2mm) board.

Additional power ports (two 6-place headers)