I've always wanted to play as a ninja in AC. (ninja's are the assassin's and samurais the templars or something like that)

But there are a lot more era's that can be covered.

The wild west aka red dead

In ACIV there was an Abstergo email with possible locations for the next Abstergo Entertainment project.

[...]
Patrilineal Line:
Fifteenth Century - Italian Renaissance
Sixteenth Century - Ottoman Empire
Eighteenth Century - American Colonies / War for Independence
Nineteenth Century - New England and American Midwest

Matrilineal Line:
Twelfth Century - Holy Land / Crusades
Thirteenth Century - Egypt and Northern Africa
Fourtheenth Century - Ashikaga Shogunate in Japan
Eighteenth Century - French Revolution
Nineteenth Century - Napoleonic wars; Taiwan
Twentieth Century - "Summer of Love" American Pacific Coast
[...]

http://www.shacknews.com/article/81881/assassins-creed-4-hints-at-the-future-of-the-franchise

I assume you mean this pic?

If so, there are numerous pics and texts in ACIV that hint at A LOT of possible future ACs. Egypt is just a single one of them. I don't think the possibility is very high because that is pretty much the only mention of Egypt at all.

pssst....japan

That's the one in the game. A picture was leaked of that character in Egypt and it was in game. This is the only one I can find.  http://oyster.ignimgs.com/wordpress/write.ign.com/141181/2013/12/AC5.jpg

Other way around. Cain is supposed to have been the first Templar (or at least his mark became the symbol of the Templars), and he killed Abel to steal the Apple.

That one I didn't know. Looks real but could be a muck up. I wouldn't hold my breath but I would love the shit out of it. Much better setting than 3 and 4

It's strange that the first assasination in the world (according to bible ofc) would be carried out by a templar, not the assassin.
Some may not know that Abel and Cain had a sister, Set. I'd love to see her in the game.

As long as it has boats, Im happy.

Would be more than happy with another piratey adventure.