Nigeria doesn’t seem likely to me. They have a lively, if nearly entirely corrupt, political scene. They have a great deal of prosperity relative to the countries around them. Most importantly, they have a strong memory of civil war and little desire to repeat it. While there are plenty of internal malcontents, unless the standard of living drops dramatically (and subsidized oil goes a long way towards keeping that from happening) I think Nigeria’s instability is going to continue to be fragmented.
What I would look at is Cameroon. All of the elements are there: A geriatric ruler who has brutally suppressed political life, a rapidly falling standard of living that is coming to the point where middle-class families are getting hard-up for food, and an inherently flawed “election” coming up next year that NOBODY is going to be happy about. During the last food crisis they had a round of serious civil instability, but it never amounted to much. But if economic conditions are bad and the elections are bad, people are reaching the point where they have nothing left to lose and will be willing to risk violence.
The oil could actually be promoting the likelihood of a war. If the entire country was broke, there’d be nothing but ethnic tension to fight over. But the oil fields add billions of dollars to the pot. Now it can be an issue of “why should we be sharing it with those guys?” Plus the oil brings in a lot of outsiders, who will play one local faction off against another.