I predict it will take place out of game and no matter the result will probably lead to the eventual collapse of the Empire. Alinor isn't going to be invaded successfully anytime soon. Even Tiber Septim knew he didn't stand a chance until Numidium, and there is not a single human out there right now in Tamriel with anywhere near the military prowess and strategic ability of Tiber Septim, and of course: no Numidium.
Valenwood is extremely hard to invade if the Bosmer put up a fight and we know the potential side effects of invading it. All we know about the Khajiit is they consider the Thalmor their saviours and the other nations bordering Cyrodiil are in no way friendly to the Empire, especially the An-Xileel, whose only dialogue we've heard regarding their involvement in the Empire/Dominion conflict is a potential weaponry trading relationship with the Dominion. I just highly doubt either Morrowind or especially Black Marsh are going to come to their aid. Hammerfell is unknown whether they'll fight with the Empire in the great war and no matter what happens Skyrim is going to be in an absolute mess after the events of TES 5, even if there is an Imperial victory in the Province. Thousands upon thousands of some of the best warriors in the Empire will be slain both in the Civil war and in the Dragon crisis.
I don't see how the Empire can 'win' the next great war, even if they take back Valenwood and Elsweyr (unlikely), the Thalmor will still remain prevelant in the Summerset Isles and are still one of if not the best cloak and dagger organizations in Tamriel. At this rate I wouldn't be suprised if the Thalmor end up controlling Cyrodiil and the Empire having collapsed by the next game. At the same time I could see a situation where the Empire regains strength and Valenwood/Elsweyr. But judging by the current trend, I think the former is more likely.