I think if the thalmor won the war then pretty much all of the other races would either be exterminated or enslaved and maybe the destruction of the mortal world. If the empire won the war completely then Tamriel would overall be much more much more stable and possibly most of Tamriel would be reunited like it was in the late third era..
If I was comanding the imperial forces. I would of made the first move launching an invasion of elswere. Having a navy blockading the south and have troops attacking on the east, North, and south After taking the capitals of the 2 kingdoms I would then attack Valenwood. I would have the navy once Again form a blockade to cut Valenwood from Alinor. Then invade the province from all directions. Then when The thalmor lost Valenwood it I would attack the smaller of the 2 islands in Alinor first. When I am done I will attack the southern part of the main island due to the capital being there from what I know lore wise. After taking the captial I would push up North Until the Thalmor surrender.
If I was in charge of the Thalmor force. I would pretty much use the same strategy as they did in the actual war but after taking the IC I will launch another massive offense pushing the empire out of Cyrrodil. Regroup with the Thalmor in Hammerfell. While also have a smaller army invade northen Morrowind where it is controlled by the empire. When Hammerfell and Northern Morrrowind is under thalmor controll I will launh an invasion of Highrock. Blocking it of from skyrim. I will attack from Hammerfell. When High rock is lost to the empire. I will attack Skyrim. Capturing Solitude first I would then advance closer to skyrim whiile Thalmor troops from all directions will push the empire to whiterun as well as eventually regroup there. And once we captured whitetun. Well the thalmor wins the war. Now let's see your thoughts.