I'm getting some really interesting responses here, a great level of variation in thoughts on this. As one should expect
As for my personal thoughts, I think that existing religions, especially Christianity, will continue to get more diverse, with more polytheist and atheist denominations emerging. Among these diverse sects, I think that Mormonism will become a sizable family of denominations, while there'll be a few newer sects emerging in Sub-Saharan Africa and China. Some of the former will, I believe, be quite fundamentalist, along the lines of Wahhabism.
Islam, I think will have a big shift into liberalism, with lots of reforms, although there'll still be lots of people who reject such changes and stay orthodox. Current groups like the Ahmadis and Quranists will, I think, grow and I reckon there'll be more Baha'i-style spinoffs.
Buddhism, I think a few sizable Western denominations will eventually congeal from the mass of Western Buddhists, but that it'll become pretty fuzzy at the edges, fading into atheism and new-age equivalents. I can, however, see it gaining a presence in areas such as Africa, and continuing to grow in Latin America, as well as undergoing a bit of a revival in India. Hinduism, and to a lesser extent Sikhism, I can see developing big new communities outside South Asia, composed of various mixes of 'converts' and people of S. Asian backgrounds. These will likely adapt to local cultures.
Baha'i I think will grow to maybe fifty million globally and stabilise. Maybe. Cao Dai, the Pagan faiths etc, who knows.
As for which ones I can see dying out - I can see religious Judaism becoming much smaller, although probably not all the way to dying out. Zoroastrianism I think will probably disappear. In both cases, I think this will be due to increasing cultural assimilation, growing irreligious sentiments in certain areas, and intermarriage.