I think it's going to be a rehash of FFXV, only with better world-building and better character motivations.
The WoLD pops out of a crystal inLestelumthe Crystarium, the city that is holding out against thedarknesslight and is constantly fighting of thescourgesin eaters. The WoLD then goes on a roadtrip with their True Companions bringing back thelightdarkness to world while fighting thescourgesin eaters off. Throwing a wrench into all of this is thechancelloremperor of the nation that wants to invade the WoLD's nation who does want thedarknesslight to consume the world, since that would be good for him back home. In the end, the WoLD ends up bringing the darkness back to the realm.
Okay... so I'm kind of kidding...
When it comes to the 5.0 MSQ, I feel like this is the most certain I've been on how an expansion MSQ is going to turn out when it comes to the broad strokes of it. I'm a lot more interested in what is going to take place after the main expansion is over since we can finally get around to doing more MSQ on the source. (Why yes, I do think the 5.0 MSQ is a world-building info-dump disguising itself as a side-quest. I hope to be proven wrong.)
1st Shard is going to get saved from the Flood of Light. We've got leveling zones in there and those have to be accessible outside of the Time Bubble. If a Rejoining/Calamity happens afterwards, there are four other Shards that can be Rejoined so the 1st Shard is't going anywhere. Really, the fact that it's part of the MSQ makes it be one of the most stable locations in the game.
Solus is going to try to cause the Flood of Light to finish. He wants to see how it interacts with Black Rose back on the Source, and to do that more light is needed rather then less. He's probably going to be the real antagonist of the expansion, but I don't think he'll be in danger of dying until after 5.0. I would not be surprised if he turns out to be the 5.3 trial.
Back on the Source, not much is going to be happening on the Alliance's side. Not because they aren't doing anything, but because Varis is having problems back home and is has better things to do then trying to take over Eorzea. Elidibus will probably keep pushing him to do something, and Varis is going to keep pushing back. Rebellions in the Garlean Provinces are probably only going to get worse (and I can't see the WoL leaving without telling Gaius what went down at the Parlay...). And the odds of his son turning up to get his body back are good and Varis knows Zenos taking over the Garlean Empire would be bad on all fronts. And Gaius might turn up and Gaius has strong opinions on who shouldn't be on the throne of Garleamld.
Elidibus is probably having a problem on his hands. The WoL is nowhere to be found on the Source and Elidibus is now stuck in a body, and a very distinctive body at that. He can't just leave Garlemald to oversee what plans the WoL is going to be messing up this time without letting his current plans go. Meanwhile, Varis thinks he can outwit Elidibus, someone is going around killing off Ascian bodies at the least (at worst, they are really killing Ascians, not just their bodies) and it will probably come to light that Zenos' Resonance experiment Went Horribly Right. Hopefully for him, Elidibus will be able to deal with Zenos...
Personally, I think something is going to happen in Garlemald while the WoL is away and the WoL, Scions and the Alliance is going to be too far away to do anything about it. My currently favored theory is that all the Black Rose Varis producing to unleash on Eorzea is going to get unleashed on Garlemald instead. Goodness knows that would be right up Solus' ally. Throw in the effects on the Source from the 1st Shard all of a sudden getting back in balance and... it's not the worst set-up for going to Ilsabard in 6.0