re: Which nation that is currently not "first world/wealthy", will be in 20 years or so?

The problem with India and Nigeria are that they have uneven growth. I've been to India many times, and while it has changed quite a bit, it still has trouble with its rural population. China has some of the same problems with its rural population. Nigeria will continue to have a high birthrate, which means it will become one of the largest Muslim countries in the world in 2050.

If the Middle East stabilizes, then there are some North African countries who have seen solid growth that could become first world. French companies in particular are eager to work in NA. I think there is a large Peugeot plant in Morocco that went online recently. Turkey has experienced a lot of growth since 2001, with their birth rate falling below or at replacement, which indicates a high level of development. If their political situation stabilizes (namely the refugee situation and the Kurdish situation) they could become the dominant economic power in the MENA region.