Why it is expected:
1)- lets consider the hints that kishi has given
a. symbolism between hashirama/madara
b. naruto vs sasuke at kage summit- naruto said he would have to fight sasuke
c. the frog sages prophecy
d. kishimotto said the manga ends with juubi,madara, and sasuke
2) why people want it to happen
a. since both are primary characters and are compared withing the story ( they are rivals) people like to compare them
b. some claim the Naruto vs Sasuke is by far the best fight in part 1.-so they want a rematch
c. why wouldn't you want to see the two strongest shinobi of their generation fight
d. its a battle manga- people want battle
e. people want proper closure with the manga and for some- having a battle between sasuke and naruto is necesary
3) Why it might happen ( plot-wise)
a. Frog Sage prophesized it
b. Naruto himself said he would need to fight sasuke and die with him
c. both want to be hokage- have different ideals
d. naruto is the protagonist- he should be proven to be = or stronger than his rival ( in shounen manga)
A fight between the hero and antihero is to some extent inevitable. both charcters have unyielding wills, yet both have different goals. naruto wants peace, while we don't know what sasuke wants exactly- it wouldnot be a strectch to say his will is not in line with Naruto's.
In my opinion the question should be - when sasuke and naruto fight- will it end with a winnner or with a draw, will both survive or die after the fight. -
-The best situation I could come up with is eithor
1) Naruto beats sasuke, and convinces him to work with him- imagine if madara and hashirama worked togethor, or hiruzen and danzo etc....
2) naruto and sasuke kill each other- manga ends with them being true equals
3) Naruto beats sasuke- and sasuke entrusts his eyes to naruto in order to carry out his will.