Balls, so..... I don't know what to do with this now I just can't explain it with anything except bad writing or orochimaru being a dunce, which he is not......
Orochimaru won't be the final villain cause he just don't have the power to. Unless he has some secret powers we don't know about or have some eye balls stash some where, he's just not strong enough.
if this war is last arc then i think final/last antagonist will be Ten-Tails. in end naruto with or without others help will defeat him end
if you wanna get really theoretical, this is a ninja series, so, like a ninja, orochimaru will sneak his way to be the final villain. also, i'm pretty sure kishimoto didn't think this far ahead of the naruto series when he started writing naruto. he spent so much time coming up with the perfect villain which would have feminine features ie. orochimaru. why waste all that creativity in developing a villain for a series and not have him do anything significant by the end of it? if naruto were a mini-success i think he would have ended the series with orochimaru much earlier.
also, think about the WOW factor between orochimaru and madara.
we have already seen madara bust out some overly-flashy-op moves against all kage
we have barely touched upon orochimaru's true skills since he hasn't really cast anything besides edo tensei after retrieving his arms back
and what's scarier/more interesting?
someone who is an enemy to the world (madara)
or someone who is almost immortal and may be close to obtaining "perfection"? (orochimaru)
(and no, don't say edo tensei'd people are immortal, they are still physically dead)
Immortality doesn't mean that he'd have infinite chakra supply.Madara on the other hand has infinite chakra,Rinnegan(and it's predecessor),Hashirama's cells.Power wise he's a complete beast.Orochimaru with all his knowledge can mess up but still,right now Madara is one dangerous enemy to the whole shinobi world.I don't think Orochimaru is capable of pulling something on the level of Edo Madara or even normal Madara/Hashirama for now.He might play his card after Madara is defeated or when the situation gets a little under control.
I would love for Orochimaru turn the Jyubi into his final host. Then he would have the body and the longevity to learn everything.
but he would still need to either take over someone elses body or inherit some monstrous power to be able to be a relevant final villain
it would be cool to see him become the jyuubi jinchuuriki or something while madara isnt paying attention because he has his hands full with the kages