I can see them doing something like Nazi Zombies, considering its success with COD WAW. However I dont think they would do the exact same thing or even something similiar again. they need to restyle it to draw people to there games. but dont worry horde mode lovers, im sure that they wont let you down.
P/S> treyarch needs a major selling point like a horde mode, so im pretty sure they will have something like it included.