Although T. R. Knight's George O'Malley has been gone for a long time now, if Meredith is going around talking to all her dead loved ones on a beach, he would definitely be one to show up! George's friendship had such a profound effect on Meredith, it wouldn't be surprising if he were one of the people giving her a bit of peace of mind while she's in turmoil.
Knight is listed as being in the show in 2020, which definitely lends some credibility to the idea that he could be making an appearance. Like Derek, he wouldn't have to do much other than be his usual charming self! And considering that Knight and Ellen Pompeo are such good friends, it isn't hard to believe that they would make it work.