It's just as you yourself already said: this is hypothetical and so is the answer.
However I do think teaming up with Sir Paul McCartney would have shook up the music industry. I understood that Paul & John always had the wish to write for Elvis. Another possibility is Tom Jones. They have attended eachother's parties and did sing along together several times so they knew eachother's capacilities.
And if Elvis would have had the role in "A Star is Born" a duet with Barbra Streisand would also have been a possibility.