Al Gore 2016 seems strange and unlikely, but not that strange and unlikely.
He's a known political quantity. Nobody in the current field can claim to have won three presidential elections, even if he was second fiddle in two of them and let the third victory get stolen in court.
On the other hand, his hard focus on, and specific positions on, environmental issues since then could come back to bite him in the ass. Lots of people disagree vehemently with him on the topic, and even some people who don't necessarily disagree with him might feel like he's turned into a one trick pony over the years.
Apparently he's thinking it over.