It's really hard to say, but if I had to predict the future, I'd imagine it goes something like this:
1) Khabib rematch in title fight: Khabib repeats mauling, tells Conor mid-fight that the bear cub he fought when he was 10 was a tougher opponent. Khabib then has a falling out with the UFC and leaves to fight exclusively in Russia and the Ukraine. The UFC erases his existence as much as possible. McGregor's legacy is untarnished.
2) Nate Diaz trilogy match @ 165. Diaz channels a lifetime's worth of anger and poor life choices into McGregor's face and then later his neck with an RNC. 165 isn't a real weight class though, and everyone knows Nate Diaz is really just a Super Heavyweight like Anthony Johnson who emaciated himself down to 165. McGregor's Spartan Heart is undeterred.
3) McGregor fights Tony Ferguson for the UFC Lightweight Title. Ferguson gets the nod via split decision since the UFC just didn't pay enough money to get two judges to flip an honest 50-43 into a 47-48. However, Ferguson gets stripped of the belt after refusing to defend the belt on 3 week's notice, because real champions don't duck. Ferguson is no real champion like McGregor. This sets up:
4) Jose Aldo rematch for Lightweight UFC Interim Lightweight Title. Aldo avenges the lowest point of his career and finally gets that feel-good Hollywood ending for his movie's sequel. Aldo later tests positive for suspiciously excessive levels of sodium in his bloodstream and the fight is ruled a No-Contest. McGregor's will is unbent, his valiant spirit unbroken.
5) McGregor then demands a raise in contract re-negotiations, which stall for a bit, and he decides to take a boxing match with Manny Pacquiao co-promoted by Zuffa LLC and McGregor productions. Pacquiao, unable to get his revenge on Mayweather, decides that at least he can beat up a common opponent worse than Mayweather. McGregor, going for broke, decides to try a headkick in the second round but whiffs and gets laid out cold by a Pacquiao heymaker. However, the fight is ruled a DQ win instead of a KO, and everyone knows a DQ isn't a real loss.
6) At this point the UFC decides he is finally ready to take on GSP in an Interim UFC Lightweight Title fight. McGregor misses weight, making it a non-title fight, because he's out of fucks to give. GSP gets his second stoppage in just a decade, and promptly retires. The UFC never speaks of the fight ever again. McGregor's magnificent reign lives on.
7) After contract negotiations fall apart, and looking to end his career on a high note, McGregor leaves the UFC to fight Eddie Alvarez in One FC in the biggest possible MMA fight outside the UFC. Alvarez technically gets the win via an early stoppage when Big John McCarthy stops the fight while McGregor's Irish lion heart is still technically beating in a robbery to be decried for the ages. McGregor wins again.
8) Strongly motivated and looking to go out on an a win not just in the minds and hearts of the people but also in the record books, McGregor agrees to wrestle Ronda Rousey in the WWE. He agrees to do the job in the first match in a program, but then flips the script and knocks Ronda out cold, and celebrates in the ring with Artem and Danis and finally gets his own feel-good Hollywood ending!
9) Later that night McGregor just happens to run into Dustin Poirier in a bar, and pre-emptively knocks him out with a bottle of Proper 12 in what becomes WorldStarHipHop's most viewed video of all time, keeping the streak alive. McGregor demands and receives royalties, content to end his glorious career on a winning streak.