1. It would be in a completely new region, so the developers could have a little fun making new factions, lore etc etc.
2. Ronto has already been referenced in passing in "The Pitt" and seems to be a military powerhouse.
3. We won't be seeing much of the Enclave in New Vegas it seems, so by the time 4 comes out we may be ready to kick some more Enclave butt, and after their defeat in D.C. it is entirely possible they retreated to the "Ronto" area, seeing as it is already an established power outside of BoS influence.
4. Canada holds an interesting place in the fictional Fallout history. It could lead to more information on Canada's annexation, and a lot about the economic condition of the U.S before the great war (for example, what could have happened if they did not take all the resources they needed from Canada).