Frankly I see CoD sales on Wii as being a real hinderence to Wii getting more FPS. From HVS yes, because they can have a little niche to themselves and be profitable but not from any of the big guns.
Why? Because if the biggest name in FPS can only sell 1m with no competition, how can they compete with CoD and even make a million with game like Left 4 Dead, or heck any FPS which would have zero history with the Wii base? They sell a lot less than CoD on HD consoles and would have to assume the same on Wii which instantly means they'll be luck to make 500k. Considering no history with the Wii audience it would likely be even lower. Not really worth their investment (in time more than cost).
If CoD could break 3-4m then I'd expect to sell a lot of FPS come to Wii. Not before then.