Unfortunately I've never played Doom but I think since I'm an rpg-oriented cat you have the right idea. Our story will be more story, humor, puzzle-oriented than killing oriented. The killing will be an unintended consequence but as you know, life is not always as you would like it to be and death is always around the corner. But that sounds about right. The little face or faces would be there to substitute for the 2.5d model as in some rpgs.
When I'm ready, if I have problems and post them, if you can help, it would be very much appreciated.