Elvis was the kind to set the standard. He dressed unlike any other and worked that way too. It wasn’t in his blood to follow fashions and do what was popular. So he wouldn’t feel comfortable engaging in vocal fakery just because everyone else in Vegas was doing it.

He would still experiment with new material and do cover versions of other artist’s stuff when he heard something that he liked. As you say, his show would continue to be an over-the-top spectacle. But he would stick to the formula that made him a legend, so there would be no showgirls or other distractions. His concerts always focused on him with only his fellow musicians and backing singers there to provide some additional fun. That was the way things had developed under the management of Colonel Tom Parker. Parker believed in keeping things stripped back with Elvis taking the spotlight. Parker also kept interest in Elvis high by limiting the number of interviews and press conferences.

Following Parker’s death in 1997, Elvis would have finally got himself the kind of new manager with fresh ideas that he should have had since the sixties. By then Elvis would have been firmly set in his ways. He would be willing to experiment with new material (maybe a duet with Katy Perry or someone) but the core aspects of his live performance would remain unchanged.