Does "black & white" mean only two religions? That would be simpler, of course, but might disappoint user who are accustomed to Civ4's normal 7 religions.
I know what you mean about not filling the spaces, though. Some purists don't like seeing their favourite works adulterated. However: if this is to be a modpack & not a single scenario, it must have scope for creating many divergent worlds within the style of Tolkien's world (which was only 1 possible history for Arda, without even envoking variable geography).
Do you plan to involve the eastern & southern civilizations in the role-playing Arda of the
Guild Companion? Such as the Chyan Empire, Bellakar, Mumakan, the elves of Luindor, the Empires of Urd and Womaw? A whole-world Tolkien simulation could include also the work of other hands - and those races would clearly have religions of their own.