Half Life and Halo combined would be ******. Gordon Freeman and John-117 killing hordes of inter-dimensional aliens, space aliens, and zombie hordes all at once. Its a five-way massacre! Gordon Freeman and the rebels + John-117 and ODST vs. the Convenant vs. the Combine vs. headcrab zombies vs. the flood.
I would do a mix of call of duty modern warfare 3 and minecraft that would be the coolest game to get resources, build your own base and buy and make guns to fend of zombies!!
I'm not sure what morphing two games means.... I think you meant fuse! I would fuse......the simplicity and depth of story of HALO with the darkness of Diablo 1.