The only way I see Sasuke vs Naruto being the last fight and not make an terrible ending for the series is for the following to happen:
-Sasuke will kill Madara making him the supreme evil guy of the series
-Sasuke kills someone close to Naruto to actually give a reason to hate Sasuke.
Sakura would be the best bet.
-Sasuke every now and then kicks a puppy to show just how evil is.
Still the best ending would
Sasuke and
Naruto
Epic battle.
Stalemate.
Madara comes in and tries to take advantage.
Naruto and Sasuke team up.
Madara defeated.
then become a lovers..

naruto will become the 1st GAY HOKAGE who always think of sasuke every now and then..