Not the Rift
A lot of people have been saying that they don't want to see Lana as the cause of "The Rift" and Lex's descent towards darkness and evil and badness and villainy.
I totally agree. That would really suck, from a fan's POV and overall good storytelling.
However, that's simply not the case.
Why?
1.) What is The Rift?
There is none. Yep, that's right. Clark and Lex have not been friends for ALL of S4. Neither of them trusted each other, they didn't act even remotely buddy buddy. I said it earlier, but I think they're just using each other if they seem at any point in S4 to be friends.
So, if there ever was The Rift, it happened quite slowly and is here.
It's not coming up next season because of Lana.
The Lana/Lex/Clark love triangle? It is a product of The Rift, not the cause.
2.) The S4 finale made it quite clear that Lex is Bad now.
Both Lex's radically different actions regarding the stone, and the Previouslys where Lex was quoted out of context to say, "I am the villain of the story" is evidence that Almiles wants us to believe that he's now nearly finished whatever 'journey' his descent into villainy would be.
Yes, we can fanwank that he's not a villain yet. We can argue that the things he did aren't that bad or as bad as they should be. We can argue that it's the lamest way of making him evil that there would be, that it's completely contradictory to Almiles' whole lightswitch metaphor they were spouting. And we'd be right. It is lame and it sucks.
But fact is, the show is very likely going to be writing him as Evil now. He might be the lamest evil Lex Luthor ever, but that's the way the show/writers want us to see him now.
In fact, the end of the S4 even had Lex explicitly and outright telling Clark what we all knew since S1: Lex and Clark are the way they are because of their fathers/family. I hated to see the subtetly torn away, but to say that Lana is the only reason why Lex turns is rather incomplete, and I don't believe a part of it at all.
Thus? Fighting over Lana with Clark does not 'turn him.'
So there you have it. It's not much reassurance, because Almiles' thinking is already so deeply flawed from S4, but at least we know that they're not thinking Lana is the Cause of All Rifts and Evil.
ETA:
Here's how I would write next season, going on the love triangle basis.
Lex knows that Lana is close with Clark. He thinks by getting close to her, he can get close to Clark and discover new things, while also pissing off Clark by 'stealing Lana's love and affection'. Clark, of course, thinks Lex is dangerous and no good, and always protective of Lana and his friends, he fights Lex. Lex sorta get what he wants out of the deal, and Clark learns for sure that he can't trust Lex, if there was ever any doubt in his mind.