I'd love to learn more about your cycles game engine. i remember seeing some more posts on the subject before and I can't find them now.
I'm a real-time-render assets designer and I love working with Blender and I use BGE quite a bit, but, as said before, it is too old and doesn't meet the minimum standards of a modern game engine, so i use Unity for testing and finalizing my designs.
If you are asking about some killer features I need in game engine, they will be the things that are not available in BGE.
A proper terrain generator with proper texture mapping that can be painted on the terrain (and I'm not talking about the existing texture paint in blender, I'm talking about terrain asset like in Unity)
A proper sky system (At the moment I'm using a textured sphere for sky texture, sun and two hemi's for lighting which works OK but I'm longing for a sky system in Blender, similar to the one in Bryce or in CryEngine).
And, of course, a better shadow casting. (The GLSL shadows remind me of games from late 90's)