No chance in Oblivion could a bunch of pointy eared elves have a chance at that.
They couldn't even take Hammerfell. I doubt they could take Skyrim. Maybe Morrowind because of all the other crap that has been happening to those poor cursed Dunmer... but Blackmarsh? No way.
They might be able to take over Cyrodiil. Possibly High Rock.
They might, MIGHT manage to break through the legion and seize Cyrodiil initially but that's as far as they would go. Hammerfell and Highrock reinforced by Skyrim would promptly massacre and drive them back. And even though they might manage to overrun the province I can't see them taking IC again or the Northern cities, maybe Leyawin and Bravil.
Their initial victories would be short lived however and cost them the whole of mainland Tamriel IMO. They don't have the military resources for a prolonged war which is why they depend on undercutting the provinces and sowing dissension.
So no, no victory outside an initial hollow one.
The Redguards would most likely destroy them immediately, if they'd try to land on Hammerfell. As for Cyrodiil, I think they could shrug them off for some days, but they would soon need help of other provinces.
I hope so. It would make an interesting TES VI if it was in a set in a Dominion dominated (whoa!) setting.
Hammerfell was devastated by the last war and is in no shape to face the Dominion alone.
Doubt it. Despite all the petty in fighting between the human races there is one thing they can all agree on. **** the dominion.
No. No matter which side wins, I'm pretty sure they couldn't take all of Tamriel on their own. And if they did, it wouldn't last long. It took a divine blooded god/Emperor CHIM boy with a Robo god and a divine prophecy to do it last time. The most they may be able to accomplish is set the road for a long lasting Dominion and kindly be thrown from power internally so the Dominion can prosper independent of Empire rule.
Since this is a Bethesda game the answer is no Humanity will win out, but if this were real people at war it's more likely. Since Humanity is currently too busy fighting itself instead of focusing on them then they could utilize their spies and do what they do best. It may not be very likely even in that scenario but possible.
Which half of a province being devastated is quite a lot.
I think Hammerfell could do a hell of a good job beating the Dominion back. Skyrim, too, considering the terrain and climate gives a massive advantage to the nords that would be defending it. Black Marsh would eat the Dominion alive, no problem. Morrowind, as it stands, would have major problems with the Dominion and would likely fall. Cyrodiil would likely fall, too. High Rock would likely beat back the Dominion, especially if they team up with Hammerfell and Skyrim.
In general, I think a fully-united Empire would stand a chance at defeating the Dominion. It'd be impossible for the Dominion to hold all the provinces at the same time, and if they pick and choose which ones to defend, they wouldn't have utter control and might slowly lose ground.
If, theoretically, Valenwood or Elsweyr were opposed to the Dominion, either one would easily stand their ground against the Dominion. Valenwood, like Skyrim or Blackmarsh, is just too hard to invade. To a lesser extent, Elsweyr is, too. The biggest problem with capturing Elsweyr is just how formidable the khajiit army should be, though.