We did say that we would be releasing the product in its polished state with current feature set before going off to add in the extra features, Religion is the only exception as it currently has hooks in the game - it was truly meant to be a fundamental component, obviously the Gods would have something to say about an Ancient Evil arising, and of course as you are essentially a former God it would make sense for you to be able to spread your faith, deceitfully or otherwise, once more.
The added time, which is not exceptional given that so many of its components do already exist, will absolutely be worth it. As for any other stretch goals, we would be moving all the way to having a fully functioning Early Access release - polished and all bugs fixed - before rolling in Rival Old Ones and other features, which would then be given to our Beta Tester tier players to help us develop and polish properly.
I absolutely understand your concern, so many games have been huge disappointments - and I remember being so excited about Spore and its revolutionary potential.... then playing it for 3 hours and never looking at it again. That Which Sleeps, like DF, is very humble at its base - allowing us to create a lot of depth without the programming and testing being excessively complex. We did it that way so we could make a game with this many options, one that everyone can mod to do exactly what they want without any difficulty.