The problem with Mormonism is that it only exists among a very limited group of people, hardly deserving comparison to the world religions featured in Civ- religions likes Christianity, Buddhism or Islam cut across ethnic and linguistic boundaries, while Mormonism is overwhelmingly limited to a specific region and a specific ethnic group- Anglo-Saxons (in the same sense as it is used in "WASP", rather than in the historical sense). Even Hinduism, which is largely limited to India, does this- because we in the west tend to think of India as a single country due to it's past as a British colony (even though the existence of Pakistan and Bangladesh proves this false), we forget that it is in fact quite diverse in terms of linguistic and ethnic groups.
Of course, this forces us to acknowledge several religions already in the game which have similar issues- Judaism, Taoism and Confucianism. Firstly, Judaism is practiced almost exclusively by a single ethnic group, and it's inclusion in the game is more an acknowledgment of Judaism's indirecy influence via Christianity and Islam, rather than it's influence in itself- there are, after all, many more Sikhs than Jews. Secondly, Taoism and Confucianism are similarly limited to what is largely a single ethnic group, Han Chinese. In this case, it has to be assumed that their inclusion is due to the great size and influence of this particular ethnic group and their influence on Chinese culture, as well as possibly being an attempt to make the game appear less Eurocentric.
If a new religion had to be chosen, I'd put my vote down for Zoroastrianism. The only other religions of real historical significance tend to be too regional, as with Shinto and Sikhism, and too tied into a single culture.