An undefeated record of 28-0, losing just one round in his entire career and dominating every opponent that he's fought. If we talk about records there's hardly any unblemished as Khabib's. To add to this he's a humble human being who doesn't get involved in trash-talking and treats the sport of MMA as a "Respect Sport".
But when we talk about the greatest of all time, the "GOAT" as they say, somehow Khabib's name is lost in that discussion. He is considered as one of the greatest for sure but the top spot is usually reserved either for someone like a Jon Jones or a Georges St-Pierre.
GSP was himself known to be a class act, a Two Division Champion, and had beaten every opponent he faced. Jon Jones, on the other hand, is the enigma of the sport that won the title at a ridiculously young age of 23 and has never been beaten since.
Khabib, on the other hand, has just a handful of title defenses and arguably hasn't been at the top of the lightweight division for that long because of one injury after the other at the early stages of his career.
Khabib is already in his 30s and has expressed the desire to call it quits and leave the fight game for good. If he ends his career after 2-3 fights with an undefeated record then for sure he will be considered as one of the greatest to ever do it. But even then, he wouldn't have defended his belt as many times as a Jon Jones or a GSP, nor would he have taken the risks of moving to different weight divisions and trying to become a two-division champion. With his record, he would surely be considered the greatest Lightweight of all time, but will he be considered the GOAT? It's highly unlikely.
For Khabib to fit that final piece of the Jigsaw puzzle he needs to move to Welterweight, take some risk and fight for the title. If he's able to do that then there's nothing left to prove for him. He can simply take the two titles with a big paycheck and ride into the sunset as the Greatest of all time.
Will he do it? It's hard to say because he has never really expressed a desire to move to different weight divisions and the current Champion in Kamaru Usman can be a huge challenge for him because of the size difference. But in many ways, that's where the attraction would be. For a change, Khabib would be an underdog going into the fight and many would expect him to lose. This is exactly what legends are made of and this is exactly what could tilt the GOAT debate in his favor. If Khabib does move up in weight and can win the Welterweight title then he would be, in the eyes of many, considered the GOAT of MMA.