Morrowind is in no shape to do anything, between the eruption of Red Mountain and the Argonian invasion the Dunmer likely do not even have full control of their own borders, nevermind the ability to assemble anything resembling a task force to help the Empire. As for the Medes and the Empire, they lost against the Thalmor with a fully united empire, including Skyrim and Hammerfell, I highly doubt that the current Mede empire, after the bloodletting in Skyrim and Hammerfell seceding, will be able to reasonably resist the Thalmor should they come a-knocking. Theres only three things that could save the Mede empire. Either the Empire gets another ruler on par with Tiber Septim, the Aldmeri Dominion breaks apart from within, or the Argonians get involved, and it may require two or even all three of those things happening.
Then there is the whole issue of MK cryptically saying that the Thalmor 'win', but the devil is in the details. The Dwemer also 'won' in their war against the Chimer, they just never anticipated that their desire to become god would result in them being physically incorporated into the Numidium. c0da seems to indicate that considering that the Thalmor are mentioned only in passing, and that they 'do not exist' anymore.