More than anything, I'd say we need a CB in January. I'd go for Winston Reid. Looks set to make the step up to a big club, could properly push Mertesacker for a starting spot, and only has 6 months left on his contract then.
If the recent trend with a star player each summer continues, then I'd want next summer's big signing to be Arturo Vidal. If we target Marco Reus instead, then we'd need to add Schneiderlin to our 'must have' list too. Either plan looks good to me - if we can make it happen.