As far as I know R* hasn't done this before. Random encounters with the last 3 protaganists in 4 or side missions with them were they become contacts on your phone. I doubt this will happen but didn't Johnny go to Los Santos before?
Not to s*** all over your idea but its already been confirmed, none of the past protagonist will return in this one. That's not to say they couldn't change their minds but as of today, Niko, Johnny and Luis won't be back.
Vegetarian- an old Indian word meaning bad hunter Elected Leader of both the 360 and PS3 GTA5 boards