Arduino-compatible micro-controller with both I2C and CAN
Many options come to mind:


Bluepill board
Literally any board with I2C and CAN buses from the STM32 series, as searchable on their website (https://my.st.com/content/my_st_com/en/products/microcontrollers.html)
ESP32


A Bluebill board is programmable with mbed, STM32HAL (CubeMX) and Arduino, all of which have either CAN libraries for the CAN peripheral or have it even built-in in the OS (mbedos). 

The Bluepill board is extremely low cost ($1.81 with free shipping from aliexpress) and features an STM32F103C8 microcontroller with 128kB Flash and 20kB RAM. Its peripherals include two I2C and one CAN bus. 





Infos &amp; Datasheet: http://www.st.com/en/microcontrollers/stm32f103c8.html, http://wiki.stm32duino.com/index.php?title=Blue_Pill

For flashing and debugging the chip you should have one cheap ST-Linkv2 clone (~3$ from china). 

For programming environments you can use:


Arduino IDE (STM32Duino Arduino Core for STM32F1/F3/F4 chips: http://wiki.stm32duino.com/index.php?title=Boards_Manager_package)
PlatformIO, more general purpose and more powerful (https://platformio.org:443/boards?count=15&amp;filter%5Bmcu%5D=STM32F103C&amp;page=1&amp;sorting%5Bname%5D=asc)


As frameworks you can use:


Arduino (see above)
ARM mbedos (builds upon STM32HAL, natively supported by PlatformIO)
STM32HAL (e.g. with project files for an IDE generated by STM32CubeMX)


Documentation references:


https://os.mbed.com/docs/v5.8/reference/can.html
https://www.st.com/resource/en/user_manual/dm00154093.pdf (Chapter 8)
https://github.com/Locoduino/Commanders


8 Bit microcontrollers from STM8AF series have a few micros with 1 I2C and 1 CAN bus: See http://www.st.com/en/microcontrollers/stm8af-series.html?querycriteria=productId=SS1583



Ready buyable evaluation board: http://www.st.com/en/evaluation-tools/stm8a-discovery.html (you want the STM8AF5288 one)

The toolchain changes then, of course. See https://my.st.com/content/my_st_com/en/extended-query.html?querycriteria=productId=LN1759$$associatedTo=SS1584