At this point, the Empire is in the midst of full scale collapse. All of their sane, reliable leaders like Regula and Varis are long gone, leaving just nutjobs like Zenos and Valens who can't/won't rule competently and are just going to end up bringing ruin to everyone. Most of the Empire's territory isn't loyal to the empire, just provinces that are ruled by fear, and are now taking the opportunity to rebel and break off. Basch just lost control of Dalmasca and now he's losing control of Bozja too. Apparently the Empire's military and industrial infrastructure has been wrecked/is inadequate to maintain the Empire, as Valens and Basch are going on a quest to obtain superweapons in the hope that they will miraculously attain victory (similar to how during WW2, when it became obvious that the Japanese's and German's military and industrial capacity was utterly, utterly dwarfed by the Allies, they began experimenting with different exotic weapons in the hope of trying to find SOMETHING that would turn things around). Even then, the Empire can't hope to quell the rebellions when their own homeland is in the midst of civil war, as people like Lord Nerva try to take over and carve out their own territories. There is also a mass defection taking place. Remember, most of the Empire's military is made up of conscripts who were taken away from their homelands. You even have Garlean scientists following in Cid's foosteps and defecting to the Eorzean Alliance too.
At best, Zenos may be able to reassert control in the capital, but it just doesn't have the numbers or the industry to quell the rebellions and maintain the empire's borders, let alone fight the Eorzean Alliance. But that's not his goal anyway. I expect he's just going to reassert control in the capital and then direct the empire's remaining engineers and scientists into trying to do something to the lifestream to trigger the sound and reenact the Final Days. Otherwise he's content with everything burning down around him.
We're already dealing with Basch and Valens in the Bozja and Werlyt questlines in Shadowbringers. By the time we reach 6.0, most occupied Garlean territories will be in open revolt and the Garleans there will have been overthrown or forced to withdraw to the empire's homeland, which will be controlled by Zenos. We might pass through the former occupied territories in 6.0, maybe fight a few holdouts or convince them to surrender. Then we will go to Garlemald's homeland. It'll be more like the Allies storming Berlin; there will be a few bad guys left who will fight to the end, but for the most part it's pretty much over and it's more of a mop up and restore order operation. Zenos will flee to to the lifestream, where the Source of the Sound is, or go to the Moon to obtain Zodiark's power. We will push for either Lord Nerva (assuming he doesn't die offscreen to Zenos) or Gaius (assuming he doesn't die in the Werylt questline) as Garlemald's new leader. We'll have to fight Zenos' regime for sure, but I don't think it'll be a hard fight like the Ghimlyt Dark.
I expect this to all happen by the first half of 6.0. The rest of 6.0 will be about trying to fix the Source of the Sound and stop the Final Days, with Zenos awaiting us at the end. We will get Sharlayan's help and the Scions will devise a plan to go to the Lifestream and fix the Source of the Sound. The devs might bring back the Presence of Silvertear Lake mystery from 1.0 and recontextualize Silvertear lake as being a cavern to the center of the world, so we will travel down that to reach the Lifestream.
The WoL will finally get to 1v1 Zenos again... but will lose. But that's okay, but the WoL isn't about being strong like Zenos; his strength is in his friends, so the Scions or an 8 man party will defeat Zenos. Then the WoL and the Scions will fix the Source of the Sound and stop the final days, and everyone lived happily ever after.
6.X patches will be about seeing what happened to Aldenard after. We'll get to see the struggles of the former occupied territories and Garlemald trying to recover and reestablish themselves. We'll get to say goodbye to the cast of characters we met during 2.0-6.0, and that will be the end of the Aldenard story and those characters. Further expansions will be self contained stories taking place in completely different lands with completely different casts of characters, like in Merycadia, the New World, other shards, or the original timeline.