I read the "After the Fall" novel and I think that would make a great Season 6
Some changes:
Like everyone is saying, Willow would separate Fred and Illyria but the Tomb that she was locked in was destroyed/lost and Willow must find another vessel to put Illyria (living or a totem/artifact)
Lorne would still kill Lindsey and he becomes an alcoholic, clouding his aurora reading. He becomes violent towards anyone who talks to him about his drinking, More violent towards Angel but Angel doesn't give up trying to help him
Wesley is still dead, hes still contract bound to Wolfram & Hart and is secretly trying to find a way out of his contract to be at peace, finds nothing until Illyria tells him of a way
Gunn is a Vampire, He still wants to be a hero so he ventures out of L.A in search for a powerful Sorcerer he read about in a book who can give him a soul
Angel seeks out The Senior Partners to try to send Hell back below with the help of his Dragon who can talk! hahaha (just thought Id put it out there)