Why would Lord Orochimaru fight with Sarutobi sensei? They have nothing to fight about and had a fairly good mentor student relationship. Lord Orochimaru, this chapter, essentially gave up any ill feelings he had towards the leaf village.
The arc can really only end with Naruto and Sasuke, vs Madara. Madara has both Rinnegan and Sharingan and can rejuvenate, while, at this point in time the revived kage dont have as many buffs applied to them. Unless Lord Orochimaru can use debuffs on the field or some jutsu's with wide area of effects for targetting multiple character units, then, really Madara will easily have an simple time handling any opponents. If even the allied shinobi forces jutsu wasn't enough to stop him...could first and second hokage really be enough? Konoha's Yellow Flash seems like he'd go to whichever field has Naruto, would be silly for Kishimoto not to give us screentime with both fighting together. Bee seems like he'd be captured to increase the tension and drama, and Kakashi-sensei and Obito would die hand in hand at the same time pushed to the limits.
I don't believe for a second that Oruchimaru has become complacent. He has another angle in this affair, and I beieve he wants to acuire the eye of the moon eye plan (or the rennengan). Due to him acuiring the memories of Kabuto, he obviously knows Madara is alive, yet he hasn't told Sasuke (who still refers to Obito as Tobi), or the others yet.
Sarutobi defeated Oruchimaru in the past, but only by taking his arms in self sacrifice. Therefore, I can see him being the sole person to see through Oruchimaru's ruse, thus attempting to put a stop to him behind the scenes.
For Orbito, it would make sense Minato and Kikashi would fight him (Obito being Minato's student and reason for his wife's death).
For Madara, he has the cells of the first. Who better to oppose him then the first and second? Even that may not be enough.
Finally, Sasuke wanted resolve as he entered the leaf village. He may have redemption by the end, but he and Naruto have to fight. The only reason why he's speaking to the Kages now is to strengthen that resolve. I don't see him teaming up against Madara, unless Oruchimaru sends the kages away after the fight, and again, I don't see Oruchimaru being so generous as to only summon them to chat with Sasuke.