ESP32 with ATMEGA328p/Arduino as GPIO+co-processor

[bennyslbs]

Hi,
I would like to program a ATMEGA328p from a ESP32.

I would like to use an ATMEGA328p as advanced GPIO-extender or co-processor for an ESP32.
I would like to communicate between the ESP and ATMEGA via I2C (SPI or UART if better/required for programming).
Bandwith: Low - I2C or UART is fine.
I have a I2C multiplexor (TCA9548A) that can do the 3V3 - 5V conversion for I2C. Therefore I2C is prefered.

I am using ESP-IDF.

Any suggestions on how to do it, or how not to do it?
- Which protocol to use?
- Bootloader or not in ATMEGA?
- Which pins?
- Any SW library or example that I should look into?

Prototype: I would like to use an Aduino during prototyping - this might gives problems programming the ATMEGA via the UART.

Br,
Benny