Kabuto is one of the first villain to be introduced, he's the longest running villain in the series, which intrigues me. Sasuke too, he was first introduced as a protagonist and the plot is very interesting.
So I hope the list is like this:
Tobi ---> Kabuto ---> Sasuke vs Naruto
actually, in the flashback that was presented by kushina that explained when minato fought madara, it was foreshadowed how important of a villain that madara will grow to be. "he will bring trouble to the world" i believe minato said something like that right after madara got away. this obviously implies that madara will make another trouble some appearance and become THE VILLAIN , remember this was like 10 years before the story of naruto began, so in correction to your point; madara is the longest Villain in the story (time-flow wise). back to my evaluation; minato actually proceeded to make a decision in consideration of madaras future appearance that will cause trouble to the world, this was also foreshadowing how much of a big deal madara IS , and this decision effected the main character(naruto) minato was considering letting naruto (the main character) to deal with madara, notice that no other villain was being foreshadowed at this point by a genius like minato. so the MAIN villain was desided that day when minato faught the masked man and realised his potential to bring disaster to the world.
my point: madara has been and IS the main villain since the day naruto was born but has just been behind the curtain in wait for everything to be reavealed first, im assuming this is kishi using his great skill of "suspense and unveiling of secrets and facts.
altering your point: kabuto in my eyes is a sub-villain, can interupt either the hero or the villain at the end to add complication to the story.
so i think the story will go a lil something like this
sasuke vs naruto ------> someone strong(i dont believe it will be naruto or sasuke) vs kabuto ------> a small group of elite(must include naruto) vs madara
obviously this can be wrong and go either way, e.g kabuto might be taken down while sasuke and naruto are in their fight or even before that. :shrug: