Not just any AVR MCU you like I think. Only those supported.

And then you have to accept and tolerate the limitations of the environment, as you have no word on which pins will be input/output/analog/pwm etc.

Of course this is expected to be so that there is compatibility with the extension boards and libs.

Like what Apple did only Arduino proved more successful.

Great idea for specific purposes (learning, hobby or education).

Ioannis