hi,
Of course you can make a game with a fixed camera gameplay. But nowadays, a pre-rendered background is useless, i mean a complete one.
You can still use a pre-rendered image for a distant scenery for example. For what i know of Gamecore, i seem to me that your idea is the hardest : matching shades or colors between 2d and 3d, collision detection, etc...
But if you were thinking of a special effect for your background (cartoon, handmade rendering, etc..) this can be achieved by a shader, by right now you can't add your own custom shader to gamecore

Finally,
doesn't require really deep coding.
well... the basic implementation of fixed camera will not be the hardest, maybe the transition between rooms or avoid a "ping-pong" effect when the player is just in the middle of two cameras. Those should give you some headaches
