While Sheik may not be in the newest Zelda (or newest couple Zeldas), I don't think there's any major reason to kick her out of the game. If they do, it's their decision because it's their game. But it would make sense to revamp he and Zelda to make them equally powerful.
Sheik and Zamus do look like they may have the same tilts, but I personally believe it would be sheer laziness to cut a character from the game and then create a "new" character with the same moves. Maybe Zamus's whip will be the same as Sheiks, and Zamus' gun the same as Sheik's B>, but in the interest of new ideas, I HOPE the other two are original.
On a different topic, do we have any idea yet of whether or not Zamus is her own character, or just the after final smash version of Samus?