Eventually the empire will declare war on the Thalmor, but in vain as the Imperial city will almost certainly fall without the tri province alliance helping them. The Thalmor will sweep in from Sumurset and Valenwood to take these arias, but Skingrad will prove difficult with it's huge walls and it's vampiric count. The first aria to succede will most likely be Colovia (Anvil Kvatch and Skingrad). The empire, leaderless and ecconomically hurt by the war, will collapse. This time they will have learned from they're past experiances and will capture Sentinel, the capital of Hammerfell. After being driven out and facing disasterous losses, the Dominion will attacks Hammerfell's south and western coasts. Consitering that the first cities the Thalmor will need to cut through are Daggerfall and Wayrest, with Sentinel and Orsinium nearby, they're chances don't look to good.

But this will prove a fatal mistake as the stormcloaks and redguards come to aid the brettons and orcs and they all form an anti dominion alliance. That will probably be where Thalmor strike next, since they can take it easily without the empire getting involved right away. High rock will be cut off from all imperial territory, causing wars between the petty city states there. If the stormclaoks win, they will be weakened by the war, but will probably be able to recover since the Redguards (also enemies of the empire) will trade with them and boost up the ecconomy. But there WILL be a second great war and all three factions, the stormcloaks, the imperials and the dominion, know that it's comming. Keep in mind that this is my opinion and you can disagree with it as much as you like. Here's how i think the second great war will play out depending on who will win the civil war in Skyrim.

Skyrim second great war mod