Okay, first of all,Black Marsh doesn't "hate" the Empire. Second of all, Elsweyr joined the Dominion because the Thalmor tricked tem into thinking they were their saviors. That's why I said the other 7, excluding Valenwood and Elsweyr along with the Isles. But if convinced that they could beat the elves, Argonia would aid in the battle. And as for Hammerfell (I've said High Rock earlier, forgive me), if they could just put aside their differneces with Cyrodiil aside for a short while, they could work together and defeat the elves. I'm not saying Hammerfell and Argonia will rejoin the Empire, but they all know that the Dominion is a MUCH greater threat.
How do you know? There was indication of anti-imperial feeling in both Elsweyr and Black Marsh long before Skyrim. The http://www.uesp.net/wiki/Lore:An-Xileel are in charge in Black Marsh now, they are Argonian nationalists and anti-imperial. In
The Infernal City the Argonian character says that the Nord-Cyrodiil culture the empire imposed on them never really "took." Likewise if you read books like http://www.uesp.net/wiki/Lore:The_Argonian_Account or http://www.uesp.net/wiki/Lore:A_Dance_in_Fire, it's obvious that the outer provinces tolerate the empire's presence and appreciate some of its benefits but are more or less ambivalent- sometimes quite negative, as when the imperials tried to collectivize Argonian agriculture and caused a lot of starvation.
I don't think anyone has any idea what Black Marsh will do. The Hist will look after its own, and who knows what that is.
I do hope that there will be dissenting forces in Valenwood and Elsweyr that will help fight the Dominion, but even so, none of these provinces are going to just fall in to imperial rule.