he said i am the barrier that you must overcome...even if you hate me (said that before killing the clan)
so right there hes saying that he (itachi) is there to make sasuke stronger then himself. He probably sees the potential within sasuke and so he killed the clan to help bring that out. I don't think he will encorparate the kyuubi into his plans just yet, for now i predict that itachi ansd sasuke will have a little tussle before itachi reveals the reason why he killed the clan. Then he'll get sasuke to work with him.
Also he wanted to fight sasuke alone so that they may be isolated during this whole procedure making sure no one interrupts them (aka konoha)
Also i'm sure he knows that tobi has the sharingan, cause back when we first meet itachi and kisame, kisame states
"There is another with the eye"
and
"To be able to see the other sharingan not just once, but twice"
meaning he has seen it somewhere else.
Itahci has also stated that sasuke can become the third MS user.
So all this points to tobi working with Itachi. And if we do incorparate the kyuubi into this, we know the sharingan and the kyuubi have ties with each other, and so in order for itachi's and tobi's goal to become a reality, they will need sasuke working on their side. Their goal may be not as evil as we think ether,
because they will most likely go after the biggest threat right now. whats that? its pein.
When Itachi appeared to naruto in the forest, what was he going to tell him? he could of captured him there, and we know he is designated to, but he didn't, he wanted to tell naruto something. What that is yet we don't knwo but maybe, just maybe itachi was thinking about telling naruto his plan, or part of it.
Also tobi said
"you go capture the kyuubi, as the akatsuki leader, failure is unnaceptable"
why would tobi tell pein this?
maybe its because he and itachi want to lure him out to kill him, because of all the supporters he has in the rain village.
Back to itachi and sasuke for a minute. Itachi said tho sasuke that theres only one way to acheive mangekyou sharingan, its to kill your closets friend. Why would itachi want sasuke to get the same type as him? that wouldn't be what they wanted. I think thats the third way to get MS. Itachi and tobi have different ones. And so for the third one, they would need to guide sasuke down that path. BUt kakashi has a mangekyou sharingan different from itach. he either has the same as tobi or a new version that he created, since its unlikely he killed his best fiend (cause gai's still alive and obito died years ago)
Also, in the first part of the manga we never actually see Mangekyou Sharing, only the effects. and its not till kakashi reveals his version that we see it.
Now back to itachi saying that he is the barrier. He doesn't mean sasuke killing him he means sasuke must overcome his hatered for him even though he hates him with all he has. So bascially itachi doesn't want sasuke to kill him, he wants to get sasuek to help him.