Nazi zombies is probably one of the best side games in a video game there could be. It's a shame they didn't add it in Modern Warfare 3. I hope Black Ops 2 has zombies, and ofcoure Richtofen.
If you didn't know this, Treyarch are the developers of the COD titles "World at War," "Black Ops," and "Black Ops 2." Infinity Ward are the developers for the COD titles "Modern Warfare," "Modern Warfare 2," and "Modern Warfare 3." Treyarch and Infinity Ward are both owned under the publisher Activision. Infinity Ward and Treyarch basically take turns releasing COD titles every year. The next COD title (Black Ops 2) will be released by Treyarch and will most certainly have Nazi Zombies in it (it's kind of their thing really).