you should do what you wanna do because thats what you will do good, not what is not that common in markets AT THE MOMENT (dont forget that you need more than a year to basically develop more than a demo. who knows whats common in a year?)
you are not a delivery-designer (as i am in real life
), you can be a creator wich is a totally different thing.
you can do a really great modern warfare- or zombie-style game if you add something special/personal to it or out of it, even its a good style-idea or a good story. a background is only a background, and you can divide all fps-games (not only fpsc) into only 5 categories: horror, modern, scifi,fantasy and history. there is nothing more. choose one and get something special out of it.
as said so many times before: it doesnt depend on the engine, background or techniques to develop something special. it only depends on your imagination and your will to a doable adaptation within your boundaries ... wich are not defined but moving when you are moving.
