Neither of those things are gonna happen likely.
Based on what we saw so far on the MSQ, Yda and Papalimo are the scions that will introduce us the the 4.0 expansion. That discredit anything related that we may "time travel" or that "a ramdon guy will recruit us". Also dont think this is something this could be done on this game, not involving major plots atleast. What I think is that something will happen that will push the Wol and the Scions to get more involved. Its possible that Garlemand will start a new campain to take Eorzea and we will try to make a an offensive atack before it even happen, taking back Ala Mhigo to have will let us to have acces to Isabard from land. I'm sure we will see more of this 3.5, since is almost 100% guaranteed that the new dungeon will be The Grantion, that colossal ship on Azys La.
Speaking about that skip of from 2.0 to 4.0. What it was said is that they were "th¡nking on doing", thinking, but not that they will do it. Honeslty, I dont think this is going to happen really. There still was no word about it. So for now I can just asume it will the same as with 3.0, but yea I dont see why they should lock all areas with the MSQ, but not with the progression, dungeon, etc.