I've suggested to my work that if they really want people back in the office full time, they should offer those that return a 4-day work week as a meaningful incentive to compensate for the lost time and money to commuting. Still waiting for them to implement that one...

I'd say probably a four day work week. I mean 4x8, not 4x10.

A shorter than 40 hour work week would be the biggest draw.

According to a study conducted by Zippia.com (1,000 full-time workers), the average worker is only productive for a little over 4 hours per day, with productivity capping out at 6 hours. This article on studyfinds.org references another 2,000 employee study done by OnePoll (no link given) that says "A new survey finds office workers are at their most productive by 10:22 a.m. each morning — but start to slump by 1:27 p.m."

Letting employees who commute to the office every day work 30 hours per week instead of 40 would be a HUGE draw for a lot of people. Less traffic on the commute, less "fluff" time where you're not doing anything, time to take care of personal errands during the week while businesses are still open, and I'm sure other benefits.