I think LG is likely.
I personally don't think and don't want Sony to do it, while their mobile division is less closed source than other areas, given their various business and history, I don't want them to associate with the Nexus. Not to mention Google is at odd with the 'Rockstar' consortium that Sony is a part of. Doesn't mean I dislike Sony phones, I have 2 Xperia Play.
I like the idea of Google keep partnering with 3rd party manufacturers instead of doing it in house using Motorola, it would be good for their image I think and gives varieties, although making a Motorola Nexus would probably be better for them at this point.
I wouldn't mind another Samsung Nexus even if the material is cheap, as long as they agree to put in a decent camera this time, their last two was really bad in that area. I don't think this partnership will happen again because Samsung is too big now like Apple and do whatever they want.
I voted for Oppo Nexus, but I don't think it will happen. It would make a phone with good build quality, great display and decent camera.