This is assuming the next game is in Hammerfell
The PC is just released from prision,Just like in every game(almost)...He/She is left in the outside of town with rags and a couple of septims(or Hmmerfells equivalent as they aren't affliated with the empire anymore) The recruiters from the army from both the sides swamp you(The Thalmor in the guise of Bosmeri monks) and you can say you'll join one side immediately or you can say you'll think about it. Either way you will be told to go to the respective castles to join up(Just like the civil war in skyrim)
But you can prolong this as much as you want and this will give you the option to finish any guild related quests or side quests you want to do,befor doing the MQ.
Once you start you are sent to the respective frontlines and there you prove your worth and get requisitioned by the general or the Dominion equivalent to do special missions for them,The elves path would eventually lead you to overall victory against the men and vice versa for the men.
You'll be allowed a couple of chances to switch sides in the MQ if you choose to.
The final couple of quest are to be faction neutral to stop rouge Thalmor agents trying to pull the plug on existance just for themselves and even before the Dominion wanted them(Thalmor) to..This results in the Thalmor becoming slightly weak,which leads to the empire declaring war on the dominion(which could be the expansion pack or a very large DLC with the PC leading Hammerfells forces into battle against the thalmor or against the empire depending on the ending of the MQ of the main game)