A day late but a dollar more.
I think the plan would effectively be to make only the line on the defender thicker, so that if the attackers line is still 'offside' applying this thicker margin of error means that there is a bigger benefit of the doubt in favour the attacker, so it's not distances the official can't see, but pick up mistakes that should have been seen.
Add that to the introduction of the automatic VAR for offsides, which would give a beep in the assistants ear (much like goal line technology) then this would remove the delays that annoy people and take away the unacceptable randomness of bad decisions. With so many games decided by a single goal, the impact of offside mistakes is disproportionately important on the outcome and justifies VAR where the stakes are so more important. I am sure most of Wales agrees tonight.
I agree simplifying the parts of the body would help, in fact players boots could be 'tagged' to increase ease of technology. Any rule change though needs to be workable for non-VAR games in lower leagues and amateur. Feet only, seems workable, and serves the original purpose of preventing goal hanging.