How to expand INPUTS on an arduino, shift register like?

Using Arduino Project Guidance
#1

This one came back to hunt me after I left the same issue unsolved a year ago.

I have a slave microprocessor relaying commands trough a serial line. Outputs are sent the usual way trough a shift register, but so far i am using the physical pins as inputs.

Any alternatives?

#2

74165 is a parallel input serial output shift register. Like a 74595 in reverse.

#3