I'm a lifelong Elvis fan, and one of the things I've often thought about is, what if Elvis lived into the 1980s and 90s. What kind of music would he have recorded? If you look at his evolving styles of the previous decades, it may give you an idea of where he was headed. When he started out at Sun Records in the mid 1950s, he was pure rockabilly: a simple blend of blues and country. By time he signed to RCA, his repertoire expanded in favor of harder, blues based rock 'n roll and doo-wop inspired ballads.
After his stint in the Army, he took on a softer pop sound, though still laced with some harder blues. His first post-Army album, Elvis Is Back! is one of the greatest rock albums in history, highlighted by a pair of blues tracks, "Reconsider Baby" and "Like A Baby". During the "Hollywood Years" of the 1960s, his movie soundtrack albums were pure bubblegum. In 1967, Elvis got back to his blues based rock 'n roll roots at a recording session that featured Jerry Reed on lead guitar. The next year, he made his famous "1968 Comeback Special" which highlighted blues and gospel performances. He followed that in the next year with what could be considered a pure soul and blues album, From Elvis In Memphis. From there he went to Las Vegas with a Country rock sound. As the 1970s rolled on, his sound became more Country, although he still dabbled in some isolated blues style tracks such as "If You Talk In Your Sleep" and "Got My Mojo Working". By the time of his death in 1977, America was at the height of a 1950s revival, and Elvis was releasing many 1950s inspired tracks, such as "Little Darlin'", "Way Down", and "Pledging My Love".
Also gone would be the jeweled caped jumpsuits that signified the 1970s. In the 1980s, he would have gone back to performing in bright color sport coats, as he did in the 1950s, and would have trimmed his hair and sideburns back to the style he wore on the 1968 Comeback Special.
In the 1990s, I think Elvis would have moved on from his rhythm and blues sound, and would have taken on more of a Country pop sound that became popular in that decade under the brand name "Young Country". But the difference with Elvis would be, I would think, he would make it a little more bluesy and perhaps even have a slight grunge edge, unlike the typical "Young Country" artists of that era. He would let his hair go slightly gray with shorter 1950s era sideburns, and his sport coats would be in darker colors.
By the late 1990s and into the 2000s, I think Elvis would be performing in tuxedos and his sound would be more middle of the road pop, but still with a slight rockabilly or blues edge to it. I think he would also be embracing the doo-wop revival that began at this time, and is still going today.