In every TES game there are the Towers. Some examples of these are the Red Tower in Morrowind, Snow-Throat in Skyrim, and Ada-Mantia in Daggerfall. If this pattern is to be followed there are only two Towers left in Tamriel. These are the Crystal Tower in the Summerset Isles and Green-Sap in Valenwood. As well as these there is one known one in Yokunda, Orichalc. These are only the known ones and it is very possible that there are more, especially in Black Marsh as it is quite uncharted by Men and Mer in comparison to the rest of Tamriel. Some have also been destroyed by the fourth era, namely the Crystal tower, which excludes the Summerset Isles and Orichalc which excludes Yokunda (which wasn't really a possibility to begin with). These have been destroyed by the Thalmor to reach immortality. If there is one (or more) in Black Marsh, the Thalmor would have issues destroying it, or even getting to the heart of the province, as Tiber Septim did in the Second Era (well, he just avoided the interior but you get the point). So, if what I have seen is correct and I am not missing anything crucial, the most possible location for the next game is Valenwood, followed by Black Marsh.

No.

Show me on shred of evidence that the Thalmor are deactivating Towers.

We can all put on our tin foil hats and extrapolate Mankar as being a part of some grander Thalmori plot, but I kinda like to think he was just a bugger doing his own thing and actually pulling it off to some extent. Shoe-horning the Thalmor into everything sorta makes those threats lose a lotta water when there was just a BIGGER bad manipulating everything.

Beyond that, the Thalmor (Or at least their ideology) was festering underground, stewing in their own angst until the Empire finally collapsed, where they begun a motion to reclaim power. Kinda hard to pull off the Towers still being important, now that Martin seems to have rendered Mundus being dependent on them moot. Adamantine seems like the only one still worth something as it stands.

Green Sap has been brokenhttp://www.uesp.net/wiki/Lore:Aurbic_Enigma_4:_The_Elden_Treein the first era. Orichalc was probably destroyed when the Redguards destroyed Yokuda. Red Mountain was deactivated by the Nerevarine, Crystal-Like-Law fell to the daedra during the oblivion crisis, White Gold seems to have been broken by Martin, and we don't really know whether Snow Throat was deactivated. The Sload have a tower (or sort-of-tower) as well which fell and then was rebuilt. It is probably still functioning. Some say the Khajiit are a tower...I suppose you could suggest that the void nights might have been a Thalmor plot to mess with that. Crystal's fall I could perhaps also see an argument for Thalmor involvement since the timing is quite suspect. I guess if we see them make a move for High Rock, it might be indication that they have such a goal. It would be ironic, considering the Towers are very much an Aldmeri institution.