However, I'm not sure he'll actually be turning up at the Bo5A. When I read the quote below, I did not take it to mean he would be showing up. When he talks about a "character in the Five Legions War or something which I'm meant to understand", to me he just sounds like someone who isn't 100% down with the details - i.e. he could simply be getting his wires crossed. When he talks about the Necromancer not being in "the original Hobbit", he's just talking about the fact that the whole Necromancer plot has been lifted from the LotR appendices. So yeah, I wouldn't fret too much yet, I wouldn't take this quote as conformation that the Necromancer will rock up to the Bo5A.
"I'm playing Smaug through motion-capture and voicing the Necromancer, which is a character in the Five Legions War or something which I'm meant to understand," Cumberbatch told Empire. "He's not actually in the original Hobbit. It's something [Peter Jackson]'s taken from Lord of the Rings that he wants to put in there."
Arwen fighting at Helm's Deep. Sauron coming out in full armor at the Black Gates and fighting the armies of the west. Both scenes, thankfully, got cut out of the movie in the planning stages. Hopefully, the necromancer fighting in the battle of the 5 armies will be an idea cut as well. The battle is fine as written! I've just finished the Hobbit and I could just picture the elves and men about to go to war against the dwarves (after a very emotional verbal attack on Bilbo by Thorin), then the goblins and wargs attacking, the dwarves and elves and men deciding to fight together, Thorin leading a dramatic charge from the mountain, the eagles arriving. I think the scenes will be amazing. Why add Sauron? To make it more like the battle at the black gate? I guess the nine riders will be next? Trolls, perhaps? Ugh!