how do you think twilight would change if the cullens were on better terms with the volturi throughout? if carlisle and aro kept in regular, friendly contact, even?
The Bella thing would become much more awkward.
In canon, it was concievable that Aro would never find out about this, certainly not until Bella was already dead. Carlisle hadn't been to Volterra in hundreds of years, the others had never gone, Edward had his suicide scheme but that would be the only reason they would ever come into contact.
The Volturi, in Twilight, were a very distant bunch that Edward thought about mostly offhand. Edward never really took them all that seriously until New Moon happened.
In this universe, they drop in, Carlisle's kept in regular contact with the man. That's going to change things.
Alice and Jasper never show up. The Cullen coven would be taken off the table for Alice as she would see that, by showing up, she'd get into contact with Aro which is the last thing she wants.
As for that matter, the Cullens may not come back to Forks. Through the Cullens, Aro knows the shapeshifters exist and may have even made contact. That would make it much less likely that the Cullens would ever return even if they do call Billy to ask if it's okay.
But if they do show up...
Well, actually, Bella dies.
Without Alice, even if Edward chooses not to eat her as he did in canon, then he has no idea she'll run into trouble in Port Angeles as that was instigated by Alice's vision.
Of course, Edward could be following her, especially as Peter and Charlotte aren't there for Edward to visit with, in which case Bella is saved from her grisly fate.
However, sooner or later, Bella dies as Alice never tells Edward that he's in love with her. Without Alice, turning her into a vampire is so far off the table it's not even mentioning.
And there's only two doors for Bella Swan.
And then Aro gets to hear all about the time that Edward ate this poor teenage girl and tries to be sympathetic. He is, however, mildly upset when he hears that Edward couldn't read her thoughts and ate her anyway.
"I would have turned her!" - Aro over the telephone.
