There is a related question In a non-life-threatening emergency, can a female doctor do a hospital procedure on a male or vice versa?. The answer to that question, addresses limited situations, and I don't think answers my question, here.
Can a male use a female for a massage or female use a male for massage? I would be doing this at home when my wife is present or in a spa where there are other customers, so I don't think that are issues of yichud.
By "massage", I'm addressing two aspects. Some people have massage for relaxation or to feel good. Others require massage therapy as part of chiropractic or other muscular or physical problems. (I am unfamiliar with the various massage techniques, so I can't reasonably ask about how halachot may apply to the various massage techniques.) Is there any difference in halacha if the reason for massage is medically necessary or for pleasure?