Personally, what I would like to see is two branches of CoD. For example they could make Call of Duty 5 in WW2 seeing as IW have proved they are able to handle WW2 extremely well but they should also work on Call of Duty Modern Warfare 2 because once again, that is a genre that they have close to perfected.
This is a great idea. But if they do two branches, it will certainly do like others and develop two differents engines, so... they can continue naming it CoD5-6-7-8.... 20 and put us in differents environments! (CoD1-2-4 rules!)