Zombies is my favorite, but that's Treyarch's baby. I'm hoping for a return to Spec Ops. I'm interested to hear other ideas, but the standard aliens, robots, and dinosaurs isn't for me. Extinction is already very similar to Zombies, but it covers different ground being class based and having set missions and progression. I think having another horde mode is unnecessary, and I wouldn't just mind bringing back Survival w/ Spec-Ops if that was the case.
The only thing I don't like about spec ops is that it doesn't add a whole lot of value to DLC. I completed all the launch missions on Veteran and got the Survival and Spec Ops titles. However, I barely touched the DLC spec ops because they were not that unique and forced perfection to get the achievements associated with them. There's no question that Zombies or something more complex is a better value for DLC.
In all honestly, I barely touch Extinction, and I still enjoy Ghosts. If the multiplayer is done well, then I won't care what side mode is introduced in Advanced Warfare.