His father wanted him to either fight GSP (we know GSP will not fight Khabib) or Floyd Mayweather Jr.
I remember Khabibs father wanted that Mayweather fight badly, so if Khabib were to comeback it would likely to fulfill his fathers wish of either fighting GSP or Mayweather
and I think Mayweather vs Khabib is more likely in a boxing match, since Dana White works with the middle east and Floyd and Dana are in a business relationship and on good terms now.
Floyd also said he will not fight any boxers anymore and his health is more important he said he will only fight UFC fighters in pro boxing 12 round bouts or Japanese MMA fighters in exhibitions.