To be GOAT, I think he would need this roster.
1. 15 GS, one over Sampras.
2. Roland Garros, which Sampras never won, earning him the Career grand slam.
3. 6 consecutive Year end number 1.
4. One more Wimbledon this year (breaking Borg's streak of 42), or two over his next 3 years, matching Sampras' 7 total Wimbledon titles.
5. One more USO title to match Connors and Sampras.
6. Winning the AO this year to best Agassi in consecutive AO titles and tying him for total AO titles.
7. Calendar year grand slam to best Laver.
8. 19 super nine tournaments, or whatever they decide to call them now.
9. total career win ratio of 82 percent (matching Borg). He is over 80 now.
10. 100+ tournaments altogether to match Lendl and Connors.
11. 7 years in the top 3 to match Sampras' dominance.
12. 300 weeks career total number 1.
That definitely wouldn't be enough, add to the list:
13. Break Nicklaus golf majors record
14. 100m. world record owner
15. Win more FIFA word cups than Brazil
16. Win more Tour de France than Armstrong
17. Win more Mr Olympia than Coleman
18. Win the peace Nobel prize (optional)