Elsweyr would be a good change of setting, if done right. However, to be a bit of a devil's advocate to some of your points:
1) That depends on how exactly the Civil War gets resolved. If they really want to take a shot at the Thalmor, it may be a better idea to split Aldmeri Dominion territory by invading Valenwood instead. The Thalmor probably still have a large presence in Elsweyr. Still, we don't know a whole lot about how Thalmor occupation is going in either province. By the time of TESVI, either one may be leading an uprising. Even for any Khajiit that want independence, there is no guarantee that they will accept support from the Empire, and many Khajiit will still probably feel fairly loyal to the Thalmor who they believe brought back the moons.
2) It isn't really between Cyrodiil and Valenwood.
3) Players want all kinds of different settings. You could make similar arguments for other provinces like any desert mods (although Elsweyr has these as well, and Elsweyr has pretty diverse environments.
4) The Crystal Tower fell during the Oblivion Crisis. Whatever--or whoever--the stone is is probably not going to be relevant, especially over in Elsweyr, unless there was some way that I can't think of to fit the Crystal Tower into the story. The stone for the Khajiit could be a bunch of different things, like the Mane or the moons.
5) On the contrary, many players won't want a total rehash of the Civil War with an Elsweyr skin. It needs to have enough added to it to keep it fresh instead of Civil War 2.0. Also, we don't know what the relationship between the Mane and the Thalmor is like. We don't know much about the Mane apart except for the fact that he/she is more or less the leader of the Khajiit and some other tidbits of information.
Now, this isn't to say that there isn't any merit to these arguments. However, I think there needs to be a bit deeper reasoning behind them, because there are plenty of arguments to be made to the contrary.