Idea for my first big project

I wanted to put in an entry so that those that might be interested could view/discuss my ideas...
For my first big project I was thinking about a game that would be christian based. I have an idea where you play as a man that goes around the city doing various community service deeds and generally having an open world availability to do as you please interacting with the denizens. This character is passive and is a story-line progressor. Now at the same time you have the option to push a button and switch to a guardian angel type character that watches over the first and slay demons and other such vile filth.
It would have a feel much like the soul reaver game where you could switch between worlds and manipulate the game in each to complete tasks. But it would be more along the lines of a elder scrolls game in its open world gameplay.
Does this make any sense to you all?

What I could use some help on is how to create the two worlds that would interact together.