I can't really seen any scenario where Buttigieg, Harris, and Booker don't try again. Klobuchar giving it another go doesn't seem far fetched, especially if there is an opening in the moderate lane.
The one that sticks out to me is Warren. This has been a tough run for her, but if Bernie doesn't run, which I think is likely given his age, someone has to carry the progressive torch. Of this group, there really is no option other than Warren.
Maybe Sherrod Brown? He did almost run this time around. Not to mention that he strikes me as one of the better candidates to bridge the moderate-progressive divide. I'm not sure that Warren would give it another try, but she'll still be a little bit younger than the likes of Biden and Sanders so I would't rule it out.