I see two basic scenarios. Firstly he may try to keep his music career going, to what extent I dont know. But he may try an album or two, some singles, maybe the occasional collab. Whether he remains relevant or not is another matter, I dont see him returning to the status he once enjoyed, but he could stay a household name until he dies. Of course that depending on his health.
Secondly he may abandon music in the mainstream and turn towards something else. Maybe he tries a tv show, or hooks up with a religious personality on a television platform. Say a tv preacher gets him to sign on and uses him to draw the crowds. The preacher gives a sermon, then takes a break to setup for a bible based drama while Elvis sings some songs. Maybe the preacher and Elvis go in business together but idk. If he went the retired rocker who found god schtick route then he could again take things a whole series of different ways.
Basically he probably wont return to his earlier fame, but from there I dont know where his career would go.