John Billingsley has previously spoken about his willingness to return to Star Trek.
While on the SAG-AFTRA picket line, John Billinglsey, who played Dr. Phlox for four seasons on Star Trek: Enterprise, was asked by the Star Trek: Strange New Worlds team how long he thought Denobulans lived. Billingsley told Trekmovie's podcast team that he responded with "‘Long. Long, long, long. Definitely long enough to be on your show if that’s what you’re getting at." The way Billingsley sees it, even if his character was dead, Dr. Phlox had a son so Dr. Phlox, Jr. could appear on the show, and "he is the spitting image of his old pop."
Clearly, Billingsley is interested in returning to the Star Trek franchise, and there is no reason why he couldn't since the Denobulan life span wasn't established during Star Trek: Enterprise. It's been less than twenty years since the actor played that character, and the rubber head would be the same so it's entirely plausible that's why he was asked about the lifespan of his character.
In the meantime, what does John Billingsley think Dr. Phlox has been up to since he left Enterprise?
The actor had some clear ideas about what Phlox would be doing in his retirement years.
"“I think he’s writing a series of very successful sex manuals. I think he’s become like the Dr. Ruth of intergalactic space, maybe got his own show. Maybe a lot of cookbooks too… Phlox, he was hedonistic. He was brilliant, but he was hedonistic. So, the Star Trek way, he would be doing research and curing diseases, guest lecturing at various medical yada yada yada . . . … I’d like to think that he got to a point where he said, ‘You know what? I’ve done a lot. It’s time for me to have fun, and I’m going to do some cool shit.’ I’d like to think he got to enjoy some of his declining years, with his various wives and husbands, traveling around the galaxy, going to Risa, having a blast.”"John Billingsley
Billingsley went on to say that he would be open to coming back in a dramatic episode, though a little surprised as his character was mostly used for comedic relief on Enterprise. He wouldn't be the only one surprised as fans know him as the fun Dr. Phlox, but it would be interesting to see him in a different way, and being that he would be younger in the show, he might not be as level-headed as he was when he became the chief medical officer of the Enterprise.
Since Dr. Phlox served on the original Enterprise it would certainly be fitting to bring him aboard Captain Pike's Enterprise on Strange New Worlds, possibly to help Dr. M'Benga solve an outbreak aboard the ship or on another planet. Maybe even something to do with the Menks from "Doctor, Doctor."